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.h

@@ -69,6 +69,8 @@ gboolean shell_global_begin_modal (ShellGlobal *global,
 void     shell_global_end_modal   (ShellGlobal *global,
 				   guint32      timestamp);
 
+gboolean shell_global_display_is_grabbed (ShellGlobal *global);
+
 void shell_global_reexec_self (ShellGlobal *global);
 
 void shell_global_format_time_relative_pretty (ShellGlobal *global, guint delta, char **text, guint *update_time);