Major fixes for the Krita filtering system

Now there is no preview mask thing. All the preview is generated
by the strokes system is absolutely the same way as it is done
for real filtering. This effectively fixes two things:

1) Filter preview for masks
2) Fixes a crash when pressing OK while the preview update is running

The patch also removes the KisFilterHandler thing, because it made
switching filters in the filter dialog fail (you could switch them,
by it didn't have any effect). Now its work is done by a simple
QSignalMapper.