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/config.h

@@ -149,6 +149,13 @@ struct Config {
      * between two workspaces. */
     bool workspace_auto_back_and_forth;
 
+    /** By default, urgency is cleared immediately when switching to another
+     * workspace leads to focusing the con with the urgency hint. When having
+     * multiple windows on that workspace, the user needs to guess which
+     * application raised the event. To prevent this, the reset of the urgency
+     * flag can be delayed using an urgency timer. */
+    float workspace_urgency_timer;
+
     /** The default border style for new windows. */
     border_style_t default_border;