Emit signals for GraphEdit multi-node selection and unselection

Emit `node_selected` and `node_deselected` signals where appropriate
when selecting or unselecting multiple GraphEdit nodes at once.

(cherry picked from commit c1d5c7727cc9a537ec81f007be0b0987ccac91df)

scene/gui/graph_edit.cpp

@@ -849,10 +849,22 @@ void GraphEdit::_gui_input(const Ref<InputEvent> &p_ev) {
 			r.size *= zoom;
 			bool in_box = r.intersects(box_selecting_rect);
 
-			if (in_box)
+			if (in_box) {
+				if (!gn->is_selected() && box_selection_mode_additive) {
+					emit_signal("node_selected", gn);
+				} else if (gn->is_selected() && !box_selection_mode_additive) {
+					emit_signal("node_unselected", gn);
+				}
 				gn->set_selected(box_selection_mode_additive);
-			else
-				gn->set_selected(previus_selected.find(gn) != NULL);
+			} else {
+				bool select = (previus_selected.find(gn) != NULL);
+				if (gn->is_selected() && !select) {
+					emit_signal("node_unselected", gn);
+				} else if (!gn->is_selected() && select) {
+					emit_signal("node_selected", gn);
+				}
+				gn->set_selected(select);
+			}
 		}
 
 		top_layer->update();
@@ -870,7 +882,13 @@ void GraphEdit::_gui_input(const Ref<InputEvent> &p_ev) {
 					if (!gn)
 						continue;
 
-					gn->set_selected(previus_selected.find(gn) != NULL);
+					bool select = (previus_selected.find(gn) != NULL);
+					if (gn->is_selected() && !select) {
+						emit_signal("node_unselected", gn);
+					} else if (!gn->is_selected() && select) {
+						emit_signal("node_selected", gn);
+					}
+					gn->set_selected(select);
 				}
 				top_layer->update();
 			} else {
@@ -893,6 +911,7 @@ void GraphEdit::_gui_input(const Ref<InputEvent> &p_ev) {
 						Rect2 r = gn->get_rect();
 						r.size *= zoom;
 						if (r.has_point(b->get_position())) {
+							emit_signal("node_unselected", gn);
 							gn->set_selected(false);
 						}
 					}