gdbusnamewatching: Ensure GDestroyNotify is called in correct context

The documentation for `g_bus_watch_name()` implies that the
`GDestroyNotify` for the user data will be called in the current thread
default `GMainContext`. Currently, it could be called in any thread, as
`client_unref()` can be called in any thread.

Fix that by deferring it to an idle source if `client_unref()` finalises
the `Client` object in a different thread.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>