Each tool can have its own set of properties that can be modified
through a panel on the right side.
The tools I've added properties for are:
Pen:
Thickness
Brush:
Size
Hardness
Spray:
Thickness
Density
Bucket:
Threshold
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. :^)