Implement support for mouse buttons to be used as menu shortcuts - fixes

 #1070

Also maps the mouse thumb button 1 to undo, and the mouse thumb button 2 to redo.

project.godot

@@ -266,12 +266,14 @@ quit={
 undo={
 "deadzone": 0.5,
 "events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"command_or_control_autoremap":true,"alt_pressed":false,"shift_pressed":false,"pressed":false,"keycode":90,"physical_keycode":0,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+, Object(InputEventMouseButton,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"button_mask":0,"position":Vector2(0, 0),"global_position":Vector2(0, 0),"factor":1.0,"button_index":8,"canceled":false,"pressed":false,"double_click":false,"script":null)
 ]
 }
 redo={
 "deadzone": 0.5,
 "events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"command_or_control_autoremap":true,"alt_pressed":false,"shift_pressed":false,"pressed":false,"keycode":89,"physical_keycode":0,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
 , Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":0,"window_id":0,"command_or_control_autoremap":true,"alt_pressed":false,"shift_pressed":true,"pressed":false,"keycode":90,"physical_keycode":0,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+, Object(InputEventMouseButton,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"button_mask":0,"position":Vector2(138, 9),"global_position":Vector2(147, 55),"factor":1.0,"button_index":9,"canceled":false,"pressed":true,"double_click":false,"script":null)
 ]
 }
 show_grid={

src/UI/TopMenuContainer/TopMenuContainer.gd

@@ -98,6 +98,18 @@ func _ready() -> void:
 	_setup_help_menu()
 
 
+func _input(event: InputEvent) -> void:
+	if event is InputEventMouseButton and event.pressed:
+		file_menu.activate_item_by_event(event)
+		edit_menu.activate_item_by_event(event)
+		select_menu.activate_item_by_event(event)
+		image_menu.activate_item_by_event(event)
+		effects_menu.activate_item_by_event(event)
+		view_menu.activate_item_by_event(event)
+		window_menu.activate_item_by_event(event)
+		help_menu.activate_item_by_event(event)
+
+
 func _notification(what: int) -> void:
 	if what == NOTIFICATION_TRANSLATION_CHANGED and Global.current_project != null:
 		_update_file_menu_buttons(Global.current_project)