Allow KeyPress events for multiple keys concurrently.

[AlexaDeWit]

I believe doing this at the engine level would require managing an internal piece of state due to the nature of how SDL handles key events, but that it definitely could be done.

I will probably work on a proof of concept at the application level myself, and then see about offering it as a PR at the Helm level if desired.

I figured I'd open this more as a request for comment than a straight issue. What do you think, @z0w0?