implement delayed urgency hint reset

If there is a client with an urgency hint on another workspace and
switching to this workspace would cause the urgency to be reset (by
moving the focusing to the client), delay the reset by some time. This
gives the user the chance to see it.

This commit adds the possibility to configure the urgency delay timer
duration using the 'force_display_urgency_hint' directive. Also,
documentation and a testcase was added to allow for automated checks of
the intended behavior.

fixes #482

include/data.h

@@ -496,6 +496,9 @@ struct Con {
      * inside this container (if any) sets the urgency hint, for example. */
     bool urgent;
 
+    /* timer used for disabling urgency */
+    struct ev_timer *urgency_timer;
+
     /* ids/pixmap/graphics context for the frame window */
     xcb_window_t frame;
     xcb_pixmap_t pixmap;