Add type_data_ref_U() and use it in g_type_class_ref()

Add type_data_ref_U() and use it in g_type_class_ref()

The function returns TRUE if the type was previously initialized and can
be easily reused. It returns FALSE and does not take a reference if the
type is not referenced yet.

g_type_class_ref() uses this to avoid taking locks in the common path,
which speeds up object creation a lot - in particular in multithreaded
applications.

https://bugzilla.gnome.org/show_bug.cgi?id=585375