Provide (or maybe just document) a way to use Event export type

[Supamiu]

When using FmodEventEmitter2D, we have access to this really useful event picker for the FMOD Event parameter.

This allows for easy event binding and proper completion to avoid potential typos in event names, it's also a very convenient way to ask for an event as input.

In my case, I'm creating a FMODButton class using GDScript, allowing to easily have Buttons that will emit events on click, hover, focus, etc

The issue is that the best I can do is using @export_enum to provide a list of possible String values and it could be made better using the already existing picker.

Maybe this is just a lack of documentation tho, hence this issue 🙂