Replace `GlobalEvent`s with a Tyrian-like `Msg` system

[davesmith00000]

One of the parts of Indigo that I've always disliked, but that I put in long before I know what I was doing, is Indigo's GlobalEvent trait.

The advantage of using this OO-style event system is that it's easy, the drawback is that it's error prone.

The Tyrian/Elm approach of asking the user to defined there event/message type and forcing them to explain how system events become part of that is much better, and would also allow us to remove some uniquely Indigo things, like event filters.

The issue is that this has wider reaching implications at this point. Still, I think it's worth a go.