Though forward declarations of the implementations are ok, it is cleaner
to have proper header files for each variant (default, kwin, quartz
right now). Of course these new header files are not installed and must
be kept private for build only.
Add support for KWin API, for KDE on Wayland.
Unfortunately though, KWin's "pick" API seems to have failures, so I
fallback to the default color picking when this happens. This will still
not work on Wayland, but at least won't cause regression for color
picking on KDE/X11.
See also KDE bug: https://bugs.kde.org/show_bug.cgi?id=387720
