Fix hang when clicking on the user status menu in the overview

shell-global.[ch]: Add shell_global_display_is_grabbed() that
  uses the newly added meta_display_get_grab_op() to check
  for existing grabs.
shell-status-menu.[ch]: Add shell_status_menu_is_active() to
  check if the menu is popped up. Check for active grabs before
  popping the menu up. Use gtk_menu_popdown() rather than
  gtk_widget_hide(). Remove an excess gtk_widget_show() and
  some excess casts.
panel.js: Check whether the status menu is popped up after button
  release, and if it's not popped up, unhighlight the button.

Reported by Nuno Donato
http://bugzilla.gnome.org/show_bug.cgi?id=593362

src/shell-global.c

@@ -512,6 +512,30 @@ shell_global_end_modal (ShellGlobal *global,
   mutter_plugin_end_modal (global->plugin, timestamp);
 }
 
+/**
+ * shell_global_display_is_grabbed
+ * @global: a #ShellGlobal
+ *
+ * Determines whether Mutter currently has a grab (keyboard or mouse or
+ * both) on the display. This could be the result of a current window
+ * management operation like a window move, or could be from
+ * shell_global_begin_modal().
+ *
+ * This function is useful to for ad-hoc checks to avoid over-grabbing
+ * the Mutter grab a grab from GTK+. Longer-term we might instead want a
+ * mechanism to make Mutter use GDK grabs instead of raw XGrabPointer().
+ *
+ * Return value: %TRUE if Mutter has a grab on the display
+ */
+gboolean
+shell_global_display_is_grabbed (ShellGlobal *global)
+{
+  MetaScreen *screen = mutter_plugin_get_screen (global->plugin);
+  MetaDisplay *display = meta_screen_get_display (screen);
+
+  return meta_display_get_grab_op (display) != META_GRAB_OP_NONE;
+}
+
 /* Code to close all file descriptors before we exec; copied from gspawn.c in GLib.
  *
  * Authors: Padraig O'Briain, Matthias Clasen, Lennart Poettering