Port kpSelectionDrag away from Q3ImageDrag -- which was buggy beyond repair
and deprecated anyway -- to QMimeData.  By removing unused functionality
and by making use of the simplicty of QMimeData, kpSelectionDrag's implementation
is significantly simpler.

In kpMainWindow_Edit.cpp, I've added a TODO about kpPixmapFX::pixmapWithDefinedTransparentPixmaps(), that refers to a future commit :(

svn path=/trunk/KDE/kdegraphics/kolourpaint/; revision=722750