appDisplay: Use the desktop file index for app searching

Rather than scanning all apps for searching, use Ryan's new desktop
file index and the glib support APIs for app searching instead of our
own system.

https://bugzilla.gnome.org/show_bug.cgi?id=711631

src/shell-app-private.h

@@ -22,11 +22,6 @@ void _shell_app_add_window (ShellApp *app, MetaWindow *window);
 
 void _shell_app_remove_window (ShellApp *app, MetaWindow *window);
 
-void _shell_app_do_match (ShellApp         *app,
-                          GSList           *terms,
-                          GSList          **prefix_results,
-                          GSList          **substring_results);
-
 G_END_DECLS
 
 #endif /* __SHELL_APP_PRIVATE_H__ */