[xcb] Use a KSelectionWatcher to monitor the compositing selection
Review Request #128566 - Created Aug. 1, 2016 and discarded
So far KWindowSystem had a custom implementation to monitor the compositing selection using XFixes and an own event filter. This change simplifies it by porting to KSelectionWatcher to share more code. While the XFixes approach is actually better it's also fighting with Qt which might result in events getting lost. At least we have reports about the code being racy at startup.