Allow special backends to support additional features

Some exotic backends like BearLibTerminal are in effect full graphical applications, and therefore not constrained by the terminal input system.
This means for those backends, a lot more input can be detected.
I don't want to add a bunch of event keys that are only supported on one backend, but it'd be a waste to hide this feature entirely.
I'm therefore thinking of adding a custom event variant, which would be backend-specific.
It means a part of the backend (the "custom" event enum) would need to be made public. Applications would then have the opportunity to tie themselves to a specific backend and use these custom events.

Still unsure if it's all worth it. I don't want to fragment too much users between backends.