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/ImageEditor.cpp

@@ -353,4 +353,9 @@ void ImageEditor::relayout()
     update();
 }
 
+void ImageEditor::image_did_change()
+{
+    update();
+}
+
 }