Do not hard runtime depend on X11 in composite startup

Summary:
The compositor needs to claim the X11 compositor selection and redirect
the X11 windows. This of course only makes sense when having X11 support.

This change refactors the code so that if X11 support is missing the code
is not executed, but as soon as X11 support comes available the selection
gets claimed.

Also if the connection goes away the selection is deleted, though it
might be that this does not work as KSelectionOwner might call into xcb
and cause a crash. This needs to be tested once we start supporting
XWayland going away.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7504