First stab at a possible masks implementation. Not everything works (apply selection, redo'ing mask creation, display mask when the mask's extent is too small, etc), but it gives a fair idea about how it could look like. Plus, it gives you something to complain about already :P

What this patch adds:
* A mask to the PaintLayer: this is both a grey8 paintdev and a kisselection (grey8 for editing and displaying, selection to actually render/merge it and to convert it easily to a selection later on)
* Some basic functions you can do with it: create and remove it, and it is rendered quite nicely I think.
* If the edit-checkbox is selected, you can edit the mask layer instead of the current paint device (similar to what the gimp does)
* If you select the display checkbox, it will display the grey8 mask instead of the masked paintdev. This is what the gimp can do, too. (somewhat buggy still)
What this patch does _not_ add:
* Bug-free masks :P
* A decent UI bit in the layerbox. Gimp and apparently ps display a small preview of the mask in the layerbox, next to the small preview of the layer. I fear this might not be possible in 1.6, which would be sad. It would be cool if it would be possible to add, though
* A 2.0 port. But that should be easy to do once finished
* Load and Save support. Unfortunately, the masks do not get saved or loaded yet. This must be fixed...

Comments and criticism on the krita-ml is appreciated ( no flaming please ;-) )