Protect readProperty and deleteProperty in case of no X11
Read(Window)Property and deleteProperty should only operate if there is
an X11 connection available.
In addition the methods are changed to the x11 wrappers provided on the
KWin::Application as that one can support restarting XWayland in opposite
to the global methods.
Test Plan: Compiles
Subscribers: plasma-devel, kwin
Differential Revision: https://phabricator.kde.org/D7651