Add Manager Signals for Callbacks

[LennyPhoenix]

Some manager's functions return results via callbacks, which are specified by passing a callback object, and a callback method name to the function. Another alternative solution could be to add signals to the manager classes that return these results. This would allow for the yield keyword to be used, reducing the amount of functions required, but would require a bit of work to get working.
We could potentially keep support for the old callback system, allowing use of both signals and pass-through callbacks.
What are people's thoughts on this?

[JezSonic]

I think adding signals to manager classes would be better or also, as you said keep support for the old callback system, could be a nice thing. In my performance signals are easier to code in GDScript, c# and Godot GDNative using C++ (I haven't tried to make signals in GDNative using C)

[LennyPhoenix]

Yeah, I think once I finish the other things on the 1.0 board im going to add corresponding signals for each callback.

[LennyPhoenix]

Started work on this now 👌

[LennyPhoenix]

This is now fully implemented.