Rename gi.types.GIObjectMeta to gi.types.GObjectMeta

Rename GIObjectMeta to GObjectMeta reverting it to its original name prior
to commit https://git.gnome.org/browse/pygobject/commit/?id=2624bd2b
That change could have been considered an API break for anyone deriving from
gi.types.GObjectMeta since it is essentially public API. Rename base meta
class to _GObjectMetaBase which was previously gi._gobject before
commit 2624bd2b (private API).