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
cvsimport
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.
cvsimport
Define per-screen groups

This should make Xinerama-aware screens behave independently of each other
when switching groups.
WIP