Protect readProperty and deleteProperty in case of no X11
Summary:
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
Tags: KWin
Differential Revision: https://phabricator.kde.org/D7651