Two things in this commit:

a) the qpainter canvas refactoring (that, as noted before, is still buggy)
b) lots of work on the node/layer/mask refactoring. Still very buggy, but 
   there has been progress since my last commit on this topic: clicking the
   relevant buttons now do insert new layers. They are just not selectable.
   More work will follow tomorrow.

svn path=/trunk/koffice/; revision=725175