Onionskin: enhancement, code restructuration and some fixes.
Fix vector translation: org coords are 0,0 for vector layer

Colored onionskin seems finished so it's requiring beta-testing and
valorations.
- Only the selected layer is showing the onion skin.
- Code is ready for adding optional multi-layer onionskin, if
convenient.
Jagged strokes fix: added a subtle anti-shaking

Low speed brush strokes are now smoothed. I can't believe I a simple
interpolation division / 2.0 was required. How I like small changes like
this one :)
Dynamic cursor enhancement

- Now, cursor size and feather adjustments skip decimals: i.e. 1.0 2.0
3.0...

- To use decimales, combine [shift] or [ctrl] with the [alt] key.

After this last change, Pencil2D seems to be entering a new level of
usability. Step by step, it's becoming more and more appropiate for
drawing and animation seems now easier. Nevertheless, beta testing can't
be omitted after so much changes.
Resolve conflict