Add documentation for LayerView
Fix ColorPicker widget issues and close #13 for the time being.

Performance of color picker has been greatly improved.
Hue wheel now reacts to saturation and lightness changes.
Private components have been moved to implementation file.
Add some improvements to Painter class.

Drawing lines, rectangles and ellipses is now possible.
Also, Painter is now non-copyable as suggested in #7.
Update documentation for Painter, Layer and ColorPicker classes.
Merge branch 'master' of https://github.com/mkacz91/apricot
Merge branch 'master' into feat/toolactionarea
Model and Painter tweaks

Layer can only be created by AnimationFrame and AnimationFrame can only
be created by Project. This ensures model integrity. Also changed the
way Layer provides Painter.