At the moment you can paint on it with a usual brush, then click
on the "Update" property and see the filling result. A lot of things are
still to be done:

1) There should be a switch between "b/w mode" and "alpha mode"
2) The eraser button doesn't work when the mask is active (though
   you can paint with the Erase composite op and remove the keystrokes
   this way)
3) The user should be able to activate/configure prefiltering of the image
4) The filtered image should be cached using some solution with timestamps
5) There should be automatic mode: the mask updates automatically every time
   you change the original layer (the same timestamps)
6) New icons needed:
    - Update (needs rename?)
    - Show Coloring
    - Show Key Strokes
    - Icon for the mask itself (and its action)
7) The filling is extremely slow. Right now the multicut algorithm doesn't
   have the fast paths for recognizing disjoint areas without doing min-cut.
8) At the moment you cannot color-pick from the mask. To implement that
   you should write the merged keystrokes projection into fakePaintDevice.
   (btw, it will simplify decorateRect() significantly!)

Ref T372