Rename CanvasItem.update() to queue_redraw()

Affects a lot of classes. Very thoroughly checked signal connections and deferred calls to this method, add_do_method/add_undo_method calls, and so on.

Also renames the internal `_update_callback()` to `_redraw_callback()` for consistency.

Just a few comments have also been changed to say "redraw".

In CPUParticles2D, there was a private variable with the same name. It has been renamed to `do_redraw`.

editor/editor_audio_buses.cpp

@@ -182,7 +182,7 @@ void EditorAudioBus::_notification(int p_what) {
 		case NOTIFICATION_DRAG_END: {
 			if (hovering_drop) {
 				hovering_drop = false;
-				update();
+				queue_redraw();
 			}
 		} break;
 	}
@@ -967,7 +967,7 @@ void EditorAudioBusDrop::_notification(int p_what) {
 		case NOTIFICATION_MOUSE_ENTER: {
 			if (!hovering_drop) {
 				hovering_drop = true;
-				update();
+				queue_redraw();
 			}
 		} break;
 
@@ -975,7 +975,7 @@ void EditorAudioBusDrop::_notification(int p_what) {
 		case NOTIFICATION_DRAG_END: {
 			if (hovering_drop) {
 				hovering_drop = false;
-				update();
+				queue_redraw();
 			}
 		} break;
 	}