Simulate raster operations -- e.g. selection border -- using stippling.
This should make such visual elements more visible on top of the document content. Backend:

* NOT operations do not require a color since they ignore the source pixels

* All operations need 2 color hints for stippling

* Tweak color hints so that things are clear and look good

* Assert-fail in some raster-simluating methods if transparent colors are passed (not supported yet due to unprotected call to kpColor::toQColor())

* Comments, style