QgsColorButton shows a color chooser dialog on click, checkboard color when translucent, and live updates during color choosing

Hi Matthias, et al.,

Finally got time to wrap this up with new features discussed in previous pull request.
https://github.com/qgis/Quantum-GIS/pull/379
(I've closed that request)

New features include:
* Checkerboard background
* Optional live updates to button from color chooser dialog. This is very useful in Composer layouts and when editing symbology (try with editing gradients). The live updating dialog is a separate class (QgsColorDialog), which was used in QgsColorButton and in the gradient editor's list widget of interior stops.

Note: Tested under Qt 4.7 and 4.8 on Ubuntu 12.04, Mac 10.7.5, Win 7. Has not been tested on Win XP, other Linux distros/themes, different Mac versions, or under Qt 4.6.

Sorry it's just a single commit, but had issues with git and squashing everything down to one commit helped overcome them.