PixelPaint: Invoke the modified_change callback when layers change

This propagates layer edits as well as undo stack operations to whoever
needs to perform updates when the image changes.

Userland/Applications/PixelPaint/ImageEditor.cpp

@@ -488,6 +488,8 @@ void ImageEditor::set_pixel_grid_visibility(bool show_pixel_grid)
 
 void ImageEditor::layers_did_change()
 {
+    if (on_modified_change)
+        on_modified_change(true);
     update();
 }