completed-signal for coroutines with more than one yield

[Warlaan]

This is a proposal in response to issue #17280.
You can yield for completion of a coroutine like this: yield(coroutine(), "completed")
but without this PR that only works for coroutines with one yield inside, since every yield keyword returns a GDScriptFunctionState and the completed-signal was only emitted for the last one.

NOTE: there's probably something missing yet - it causes an error that I don't quite understand (Object::disconnect warns that the signal "completed" didn't exist) and I assume that referencing the previous GDScriptFunctionStates with raw pointers may not be a good idea since it probably won't make sure that the previous states aren't freed before their completed-signal is emitted.

[Warlaan]

The error was probably related to me not storing references before. That's fixed now, but I'd still appreciate it if someone had a look and told me if I am handling the references correctly.

[akien-mga]

Godot's C++ code uses snake_case for all identifiers, should be previous_state.

[akien-mga]

Could you squash the commits together? Commits 2 and 3 are fixups of the first one, so we only need the final state.

[Warlaan]

If it's ok I would use the opportunity to remove the code duplication in that place.

[akien-mga]

If it's ok I would use the opportunity to remove the code duplication in that place.

Yeah that would be nice :)

[hpvb]

Tested, the code makes sense, and I think the struggle is real :)

Thanks!

[Douglas-W-Williams]

@hpvb Any chance this will be added in 3.0.3?

[hpvb]

Cherry picked into 3.0.3