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/plugins/visual_shader_editor_plugin.cpp

@@ -6263,7 +6263,7 @@ void VisualShaderNodePortPreview::setup(const Ref<VisualShader> &p_shader, Visua
 	type = p_type;
 	port = p_port;
 	node = p_node;
-	update();
+	queue_redraw();
 	_shader_changed();
 }