gdbusproxy: Replace home-grown weak ref implementation with GWeakRef
gdbusproxy: Replace home-grown weak ref implementation with GWeakRef

The fix for bgo#651133 (commit 7e0f890e38) introduced a kind of weak
ref, which had to be thread-safe due to the fact that `GDBusProxy`
operates in one thread but can emit signals in another.

Since that commit, `GWeakRef` was added, which does the same thing. Drop
the custom code in favour of it; this should be functionally equivalent,
but using an RW lock rather than a basic mutex, which should reduce
contention.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib