Add ctrl+shift+a to instance scene in scenetree dock

Adds another key shortcut to instance a scene in the scentree dock. Complements ctrl+a to add a node.

(cherry picked from commit ea5445655ced4568a37d596cd0af71ce8c7809d4)

editor/scene_tree_dock.cpp

@@ -3088,7 +3088,7 @@ SceneTreeDock::SceneTreeDock(EditorNode *p_editor, Node *p_scene_root, EditorSel
 	ED_SHORTCUT("scene_tree/rename", TTR("Rename"), KEY_F2);
 	ED_SHORTCUT("scene_tree/batch_rename", TTR("Batch Rename"), KEY_MASK_SHIFT | KEY_F2);
 	ED_SHORTCUT("scene_tree/add_child_node", TTR("Add Child Node"), KEY_MASK_CMD | KEY_A);
-	ED_SHORTCUT("scene_tree/instance_scene", TTR("Instance Child Scene"));
+	ED_SHORTCUT("scene_tree/instance_scene", TTR("Instance Child Scene"), KEY_MASK_CMD | KEY_MASK_SHIFT | KEY_A);
 	ED_SHORTCUT("scene_tree/expand_collapse_all", TTR("Expand/Collapse All"));
 	ED_SHORTCUT("scene_tree/cut_node", TTR("Cut"), KEY_MASK_CMD | KEY_X);
 	ED_SHORTCUT("scene_tree/copy_node", TTR("Copy"), KEY_MASK_CMD | KEY_C);