Move X11-specific bits of keep above and keep below state to X11Client

Summary:
Nothing special about this patch except of the removal of code that forces
_NET_WM_STATE_BELOW and _NET_WM_STATE_ABOVE. If the client window is
compliant with the EWMH spec, then _NET_WM_STATE_BELOW and _NET_WM_STATE_ABOVE
will be in sync with keepBelow() and keepAbove(), respectively.

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27120