* only show message box on exit if the image has not been saved and is not blank

* only show message box on exit if the image has not been saved and is not blank
* set as wallpaper: now working, setting wallpaper style: not yet
* made number of undo-steps changeable through define-statement; increased to 10 (see original)
* zoom tool got its track bar (see original)

svn path=/trunk/; revision=43266