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. :^)
Instead of using a TableView to show the layer stack, let's build a new
custom widget for this purpose and give it some neat features!
This patch also introduces an ImageClient interface for Image to notify
interested parties about things happening. The LayerListWidget is the
first ImageClient and listens for "layer added" and "layer removed"
notifications. :^)
ImageEditor now supports panning (Ctrl+MiddleMouse) and scaling (Wheel)
the image. This took a lot of unpleasant coordinate math, but now it
actually kinda works and feels awesome! :^)
