Bring back Unref methods

[diamondburned]

Unref methods should definitely be restored as a workaround for gotk4's incomplete automatic memory management. It's probably the most straightforward way for applications to fix memory leak issues.

Ideally, when calling Unref, we should clean up all of the internal automatic object tracking. We should at least:

• Delete all callbacks off the registry,
• Delete the interned object itself off the registry, and
• Delete all finalizers attached to the interned object.