Add `GraphEdit` drag notifications

This commit adds two signals:
 * connection_drag_begun, which is emitted when a connection is started
   to be created by the user and
 * `connection_drag_ended`, which is emitted when no longer a connection
   is created.

Additionally `force_connection_drag_end()` adds the possibility to end
the connection dragging. If called from user code, no other connection
request signals are invoked. This is useful to add `GraphNode`s via
shortcuts while the user is dragging a connection to directly connect
the newly added node.

doc/classes/GraphEdit.xml

@@ -109,6 +109,14 @@
 				Removes the connection between the [code]from_port[/code] slot of the [code]from[/code] GraphNode and the [code]to_port[/code] slot of the [code]to[/code] GraphNode. If the connection does not exist, no connection is removed.
 			</description>
 		</method>
+		<method name="force_connection_drag_end">
+			<return type="void" />
+			<description>
+				Ends the creation of the current connection. In other words, if you are dragging a connection you can use this method to abort the process and remove the line that followed your cursor.
+				This is best used together with [signal connection_drag_begun] and [signal connection_drag_ended] to add custom behavior like node addition through shortcuts.
+				[b]Note:[/b] This method suppresses any other connection request signals apart from [signal connection_drag_ended].
+			</description>
+		</method>
 		<method name="get_connection_line">
 			<return type="PackedVector2Array" />
 			<argument index="0" name="from" type="Vector2" />
@@ -241,6 +249,19 @@
 				Emitted at the beginning of a GraphNode movement.
 			</description>
 		</signal>
+		<signal name="connection_drag_begun">
+			<argument index="0" name="from" type="String" />
+			<argument index="1" name="slot" type="String" />
+			<argument index="2" name="is_output" type="bool" />
+			<description>
+				Emitted at the beginning of a connection drag.
+			</description>
+		</signal>
+		<signal name="connection_drag_ended">
+			<description>
+				Emitted at the end of a connection drag.
+			</description>
+		</signal>
 		<signal name="connection_from_empty">
 			<argument index="0" name="to" type="StringName" />
 			<argument index="1" name="to_slot" type="int" />

scene/gui/graph_edit.cpp

@@ -600,6 +600,7 @@ void GraphEdit::_top_layer_input(const Ref<InputEvent> &p_ev) {
 									to = get_node(String(connecting_from)); //maybe it was erased
 									if (Object::cast_to<GraphNode>(to)) {
 										connecting = true;
+										emit_signal(SNAME("connection_drag_begun"), connecting_from, connecting_index, false);
 									}
 									return;
 								}
@@ -616,6 +617,7 @@ void GraphEdit::_top_layer_input(const Ref<InputEvent> &p_ev) {
 					connecting_target = false;
 					connecting_to = pos;
 					just_disconnected = false;
+					emit_signal(SNAME("connection_drag_begun"), connecting_from, connecting_index, true);
 					return;
 				}
 			}
@@ -642,6 +644,7 @@ void GraphEdit::_top_layer_input(const Ref<InputEvent> &p_ev) {
 									fr = get_node(String(connecting_from)); //maybe it was erased
 									if (Object::cast_to<GraphNode>(fr)) {
 										connecting = true;
+										emit_signal(SNAME("connection_drag_begun"), connecting_from, connecting_index, true);
 									}
 									return;
 								}
@@ -658,7 +661,7 @@ void GraphEdit::_top_layer_input(const Ref<InputEvent> &p_ev) {
 					connecting_target = false;
 					connecting_to = pos;
 					just_disconnected = false;
-
+					emit_signal(SNAME("connection_drag_begun"), connecting_from, connecting_index, false);
 					return;
 				}
 			}
@@ -740,11 +743,9 @@ void GraphEdit::_top_layer_input(const Ref<InputEvent> &p_ev) {
 			}
 		}
 
-		connecting = false;
-		top_layer->update();
-		minimap->update();
-		update();
-		connections_layer->update();
+		if (connecting) {
+			force_connection_drag_end();
+		}
 	}
 }
 
@@ -1162,9 +1163,7 @@ void GraphEdit::gui_input(const Ref<InputEvent> &p_ev) {
 				minimap->update();
 			} else {
 				if (connecting) {
-					connecting = false;
-					top_layer->update();
-					minimap->update();
+					force_connection_drag_end();
 				} else {
 					emit_signal(SNAME("popup_request"), get_screen_position() + b->get_position());
 				}
@@ -1394,6 +1393,17 @@ void GraphEdit::clear_connections() {
 	connections_layer->update();
 }
 
+void GraphEdit::force_connection_drag_end() {
+	ERR_FAIL_COND_MSG(!connecting, "Drag end requested without active drag!");
+	connecting = false;
+	connecting_valid = false;
+	top_layer->update();
+	minimap->update();
+	update();
+	connections_layer->update();
+	emit_signal(SNAME("connection_drag_ended"));
+}
+
 void GraphEdit::set_zoom(float p_zoom) {
 	set_zoom_custom(p_zoom, get_size() / 2);
 }
@@ -2165,6 +2175,7 @@ void GraphEdit::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_connection_activity", "from", "from_port", "to", "to_port", "amount"), &GraphEdit::set_connection_activity);
 	ClassDB::bind_method(D_METHOD("get_connection_list"), &GraphEdit::_get_connection_list);
 	ClassDB::bind_method(D_METHOD("clear_connections"), &GraphEdit::clear_connections);
+	ClassDB::bind_method(D_METHOD("force_connection_drag_end"), &GraphEdit::force_connection_drag_end);
 	ClassDB::bind_method(D_METHOD("get_scroll_ofs"), &GraphEdit::get_scroll_ofs);
 	ClassDB::bind_method(D_METHOD("set_scroll_ofs", "ofs"), &GraphEdit::set_scroll_ofs);
 
@@ -2262,6 +2273,8 @@ void GraphEdit::_bind_methods() {
 	ADD_SIGNAL(MethodInfo("begin_node_move"));
 	ADD_SIGNAL(MethodInfo("end_node_move"));
 	ADD_SIGNAL(MethodInfo("scroll_offset_changed", PropertyInfo(Variant::VECTOR2, "ofs")));
+	ADD_SIGNAL(MethodInfo("connection_drag_begun", PropertyInfo(Variant::STRING, "from"), PropertyInfo(Variant::STRING, "slot"), PropertyInfo(Variant::BOOL, "is_output")));
+	ADD_SIGNAL(MethodInfo("connection_drag_ended"));
 }
 
 GraphEdit::GraphEdit() {

scene/gui/graph_edit.h

@@ -267,6 +267,7 @@ public:
 	bool is_node_connected(const StringName &p_from, int p_from_port, const StringName &p_to, int p_to_port);
 	void disconnect_node(const StringName &p_from, int p_from_port, const StringName &p_to, int p_to_port);
 	void clear_connections();
+	void force_connection_drag_end();
 
 	void set_connection_activity(const StringName &p_from, int p_from_port, const StringName &p_to, int p_to_port, float p_activity);