Zoom Tool work:

* Zoom out on RMB click

* Draw zoom rectangle for drag-based zooms
- Still need to implement the actual zoom

* Drop zooming with Shift held down, where you would zoom in or out depending on whether you move the mouse up or down as KolourPaint's zooming is not fast enough for that and it was buggy anyway

* Use cross cursor
* Add wait cursor while zooming (doesn't quite work - see the TODO in endDraw())

* Add haventBegunDrawUserMessage() statusbar message