Menu model exporter: clean up the API

Menu model exporter: clean up the API

Give it the same treatment as the exporter for GActionGroup just got.

There is a wart here: the exporter attempt to re-enter GDBusConnection
when it is freed in order to cancel outstanding name watches.
GDBusConnection holds its own lock while calling the destroy notify, so
the attempt at reentrancy results in a deadlock.

We have a workaround to deal with that for now...