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.