Signal connection interface should be able to list existing functions

[QbieShay]

Godot version:
11d7738

Issue description:
The editor interface for connecting a signal is absolutely not friendly when it comes to connecting a signal to an existing callback, since you have to remember the name and type it.

Proposal:
Have an interface similar to the animation player's one for the call function track, that allows to choose between a list of methods.

[vnen]

I think this interface needs a big overhaul to make it friendly.

[Calinou]

Closing in favor of godotengine/godot-proposals#239, as feature proposals are now tracked on the Godot proposals repository.