`EwmhFullscreen` unconditionally sinks windows in response to a
`_NET_WM_STATE` root window message removing `_NET_WM_STATE_FULLSCREEN`.
Unfortunately, at least some versions of either the Gtk or GNOME
libraries send this on startup while restoring a window's last
known state, which means a `manageHook` `doFloat` will be undone.
This change ignores the remove if the window is not fullscreen.
A followup issue, #865, has been added for the follow-on problem
that the floating state of a window is not restored on removal of
the fullscreen state.
Fixes: #820
318ee92e61