Implement the urgency hint for windows/workspaces

Thanks to Mikael for bringing it to my mind. This change introduces
two new color classes, client.urgent and bar.urgent. By default,
urgent clients are drawn in red (colors by Atsutane).

include/data.h

@@ -201,6 +201,9 @@ struct Workspace {
         /** Temporary flag needed for re-querying xinerama screens */
         bool reassigned;
 
+        /** True if any client on this workspace has its urgent flag set */
+        bool urgent;
+
         /** the client who is started in fullscreen mode on this workspace,
          * NULL if there is none */
         Client *fullscreen_client;
@@ -400,6 +403,9 @@ struct Client {
          * the screen and its requested size is used */
         bool dock;
 
+        /** True if the client set the urgency flag in its WM_HINTS property */
+        bool urgent;
+
         /* After leaving fullscreen mode, a client needs to be reconfigured
          * (configuration = setting X, Y, width and height). By setting the
          * force_reconfigure flag, render_layout() will reconfigure the