[3D] add global toggle to enable stereoscopic 3D features/support
[3D] add more 3D specific cActions and move handling/processing of those to the GraphicContext
[3D] add cAction to toggle stereoscopic view on/off based on last used 3D mode. This allows to stick with a predefined preferred 3D mode.
[3D] always start XBMC in 2D mode
[3D] cosmetics