gdbus: fix race condition in connection filter freeing

If you called g_dbus_connection_remove_filter() on a filter while it
was running (or about to be run) in another thread, its GDestroyNotify
would be run immediately, potentially causing the filter thread to
crash.

Fix this by refcounting the filters, and using the existing mechanism
for running a GDestroyNotify in another thread in the case where the
the gdbus thread is the one that frees it.

Also, add a bit of documentation explaining this (and add a related
clarification to g_dbus_connection_signal_subscribe()).

https://bugzilla.gnome.org/show_bug.cgi?id=704568