Add a new function callback called GtkTetBufferCommitNotify to be notified
of changes to a GtkTextBuffer without being involved in a signal chain.

This is necessary for some situations because signal handlers may modify
the parameters as they proceed down to default handler. As such, the signal
is unsuitable for applications heavily utilizing plug-ins such as Builder
due to non-deterministic signal connection ordering.

This technique has been used in Builder for the better part of a decade
and now would also vastly help in situations like libspelling where you
also want to know about changes to the buffer right before they are
committed to the b-tree.

Fixes: #6133