This patch adds scaling function to the move tool.
When the cursor is over the lower right corner of the layer, it changes.
This is to signify that the layer can be scaled by dragging the mouse.
There is currently no preview of the scaling.
Doing a resize every time the mouse moves leads to unexpected behavior.
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.
