Blender Git Loki

Palettes: Refining based on user feedback on blenderartists:

* Palette colors can be set to active brush by L-clicking
* Palette colors can be edited by ctrl-L-clicking (regular colours can
be edited by simply L-clicking)
* Palette colors can be deleted by hovering over them and pressing
delete or X

To make this work, an extra button property was added. Also I separated
the color button handling code since it was becoming too much of its
own.