[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
[composer] Tweak logic for data defined map extents
[composer] Prevent data defined size of scalebar from being less than calculated minimum size
[composer] Flip all color buttons to QgsColorButtonV2
more const-correctness and avoid ambiguity in overloaded function
Use rounded rectangles for preview icons in QgsColorButtonV2
Flip all symbology color buttons to QgsColorButtonV2
Port some more color buttons to QgsColorButtonV2
also handle UI when manually applying filter
[composer] Ensure that data defined page size and orientation is correctly set when printing directly from an atlas. (Note that mixed orientation prints are unsupported on some platforms/print drivers)
Merge remote-tracking branch 'upstream/master'