[FEATURE] New version of color button (QgsColorButtonV2) based off
QToolButton. Features:
- context menu items have been moved to the attached menu button
- new gui widget QgsColorSwatchGrid, which displays a grid of colors
- new class for QgsColorScheme, which generates colors to show in a
  color swatch grid
- new class QgsColorSchemeRegistry, with a global instance containing
  default color schemes. QgsColorButtonV2 accepts a color scheme
  registry, to control which schemes to show in the popup menu as
  color swatch grids.
- color button can have a default color
- color button can also be quickly set to a totally transparent color
- c++ and python unit tests for all core components