Implement a dispose function for GSource

See !675 (comment 589306) for the related discussion.

Usage in GStreamer can be found here: https://gitlab.freedesktop.org/gstreamer/gstreamer/merge_requests/304

It's also a problem in two places in the Rust bindings, which I'll update once this is merged. I don't know where else but I'm sure there are other places with the same problem elsewhere.