Add support for XUrgency and matching _NET_WM_STATE_DEMANDS_ATTENTION
ewmh hint; urgencyborder is configurable.  The urgency flag will stick,
even while on a client in a non-viewable group, until the client
receives focus (where the border is reset).  Initial diff from Thomas
Adam with some changes/enhancements from me.
stray space and sort while here
we need the save-set when re-exec'ing so as to not lose State on our hidden clients
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Instead of using work area, use the Xinerama area for snap calculations;
based on a patch from Thomas Adam with appropriate adjustments for gap.
Teach screen_find_xinerama() to apply gap only when told to do so;
adjust callers.  Needed for an upcoming feature.
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Implement support for EWMH's _NET_WM_STATE_FULLSCREEN hint.

Since we already have a form of 'maximize', we need to differentiate
between 'maximize' and the new 'fullscreen' mode.  The 'maximize' mode
will continue to honor gap but now *retains* the border, matching the
'vert/horz maximize' behaviour.  The new 'fullscreen' mode supports and
follows the _NET_WM_STATE_FULLSCREEN hint, allowing the client perform
additional window modifications; in this mode, cwm(1) will *ignore* gap,
remove borders and freeze(move/resize) the client.  Additionally,
'fullscreen' mode will remember various combinations of previous states.

* default keybinding changes: CM-f 'fullscreen', CM-m 'maximize' (re-map
  as desired).

Positive feedback from a few, testing and ok sthen@
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