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`.

scene/gui/scroll_container.cpp

@@ -312,7 +312,7 @@ void ScrollContainer::_reposition_children() {
 		fit_child_in_rect(c, r);
 	}
 
-	update();
+	queue_redraw();
 }
 
 void ScrollContainer::_notification(int p_what) {