Deprecate KDecoration's AbilityColor

Nobody is interested in whether the Abilities are supported.
There is one method in KWin core checking for the colors
supported by the currently loaded decoration:
Workspace::decorationSupportedColors

This method is not called from anywhere inside KWin, but is
part of the D-Bus interface, though nobody in KDE's repository
is calling it [1].

As it is part of public API the Abilities are only deprecated
and scheduled for removal with the next big break.

[1] http://lxr.kde.org/search?filestring=&string=decorationSupportedColors

REVIEW: 105785