New feature: bitmap smudge

Finger smudge can move colors, smooth them and create colouring effects.
Depending on the length and pressure of strokes, the resulting effect
changes a bit.
Merge pull request #188 from eVoLuTiVeMEDIA/master

New feature: bitmap smudge
Merge branch 'master' of https://github.com/pencil2d/pencil