Summary:
Several effects announce a support property atom on the root window. This
change forwards the KWin::Application's signal that the xcbConnection
changed to the EffectsHandler so that the effects can respond to it.

All effects which announce a support property connect to this new signal
and re-announce the property. In case the xcb connection died (future
XWayland crashing case) it is set to XCB_ATOM_NONE by that. In case the
xcb connection got created (future delayed XWayland startup) the atom is
set to the proper value.

In addition all usages of the support properties are guarded, so that no
nonesense actions are performed if the support property is XCB_ATOM_NONE.

Test Plan: Only compile tested as we don't have XFree KWin yet

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7761