- introduce new state "boolean is_first" which tracks if the currently
drawn rectangle is the first with this instance
- cancel the widget if there was no movement when creating the first
rectangle
- undo to the previous rectangle if the user created a zero-extent
rectangle
- also undo to the previous rectangle if a newly drawn rectangle is
canceled with button-3 release
which is a replacement for GimpRectangleTool. It's a massive piece of
code and I'm not sure everyting works as it should, but it seems to do
crop stuff without any glitches.
