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

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7651