By using QWindow we can also find the TabBox window on Wayland where
the winId is not really helpful.
This also changes the elevation from Unamanged to Toplevel, so that
it is no longer X11 specific.
Result: TabBox stays on top of all windows also on Wayland.
KWin::connection() uses the property to resolve the value instead of
using QX11Info. In practice this doesn't change anything at the moment,
but allows kwin_wayland to provide an xcb connection without depending
on QX11Info.
As we cannot make xcb_connection_t* available as a metatype, the
property's type is set to void*.
