PixelPaint: Have layers and images send out notifications on changes

We use this to automatically update the thumbnail in LayerListWidget
when you draw into a layer. We also use it to repaint the ImageEditor
when the image changes somehow. :^)

Applications/PixelPaint/BucketTool.cpp

@@ -87,7 +87,7 @@ void BucketTool::on_mousedown(Layer& layer, GUI::MouseEvent& event, GUI::MouseEv
 
     flood_fill(layer.bitmap(), event.position(), target_color, m_editor->color_for(event));
 
-    m_editor->update();
+    layer.did_modify_bitmap(*m_editor->image());
 }
 
 }