Ported Fill Tool to strokes

This patch does 3 major things:
1) Ports the tool to strokes framework

2) Adds KisSimpleProcessingVisitor class which makes creation of new
   processings easier (Lukas can use it now in his branch).

3) Adds lazy fetching of the "initial" image set for the qimage-based
   tests. Now this set is shared by all the unittests, so there is no
   need to store copies of it.

BUG:316182
CCMAIL:lukast.dev@gmail.com