UNITY: Call g_type_init only with GLib < 2.36

g_type_init() is deprecated and does nothing except raising a number of ugly warnings in GLib >= 2.36, and UnityTaskbarManager::UnityTaskbarManager() does in fact call g_type_init(), so I use GLIB_CHECK_VERSION (https://developer.gnome.org/glib/stable/glib-Version-Information.html) to only call that with legacy GLib