The Undo/Redo actions now tell you what kind of action will be
undone/redone. This is achieved by adding an "action text" field to the
ImageUndoCommand and having everyone who calls did_complete_action()
provide this text.
This is in preparation to support masking of Layers. We now distinguish
between the "display_bitmap" which will be the whole Layer with every
effect applied and the "content_bitmap" which contains the actual
unmodified pixels in the Layer.
