hook a DrawingActionBroker to HeadlessWorkspace
DrawingAction: remove outdated comment
add ImportDrawing DrawingAction
add ActionBuffer.list method, useful for debugging
Replace Stamp drawing action with (new) ReadImage action.

This is mostly a way to work around the fact that stamping
is too dependent on current state to be easily model as a
an Action. Storing a whole byte array is far from ideal but will
have to do for now.
remove trail drawing image from mirrorables and use DrawingActions for testing mirroring
world not needed in DrawingActionBuffer/Runner anymore (a good thing)