FEATURE: New "Hard Overlay" blending mode

Blending mode and a hotkey it "Shift+Alt+P"
Mostly explained here: https://phabricator.kde.org/T6037

Many thanks for Radian Art for implementing and testing it!

Reviewers: Krita, dkazakov, rempt
Reviewed By: Krita, dkazakov
Subscribers: timotheegiet, kamathraghavendra, Krita

Differential Revision: https://phabricator.kde.org/D5903