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.

cwmrc.5

@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: July 8 2013 $
+.Dd $Mdocdate: November 27 2013 $
 .Dt CWMRC 5
 .Os
 .Sh NAME
@@ -125,6 +125,9 @@ Set menu background color.
 .It Ic color menufg Ar color
 Set menu foreground color.
 .Pp
+.It Ic color urgencyborder Ar color
+Set the color of the border of a window indicating urgency.
+.Pp
 .It Ic color ungroupborder Ar color
 Set the color of the border while ungrouping a window.
 .Pp