Support disconnection for type-safe signals

[ColinWttt]

Related to #1000

self.signals().my_signal().disconnect(...);
// may be?
self.signals().my_signal().disconnect_all()

[Bromeon]

Lower priority, since this can (likely) be added in a backwards-compatible way and thus shouldn't block 0.3.0 release.

Disconnecting by passing in the same function again is unreliable, especially for closures and other ad-hoc FnMut objects. Ideally we could go for such an API:

let handle = self.signals().my_signal().connect(...);

handle.disconnect();

But since we still have to rely on underlying Signal::disconnect(Callable) or Object::disconnect(StringName, Callable) APIs, we have to see whether that's feasible -- maybe by crafting a custom Callable that has the same hash as the original one and compares equal.

If this doesn't work out, users would need to pass in a custom ID (maybe through connect_builder()).

[Bromeon]

Maybe related, we should check what to do with connected signals once their object dies (e.g. through queue_free).
At the moment, connected signals panic with:

{Class}::bind_mut: access to instance with ID {id} after it has been freed

Maybe auto-disconnect is an option in cases where the object is known to godot-rust (passed in as connect_self/connect_other argument)? Or a detection + more precise warning + advice to disconnect signals on death?