Request for SDL3 Support

[hsdk123]

Hi, with SDL3 now being released for a while, this is a request for it's support.
https://wiki.libsdl.org/SDL3/NewFeatures

[mosra]

Yes, it's planned, I was already counting on it when making the new PointerEvent stuff -- in particular, exposing event coordinates as floating-point values, because that's what SDL3 uses, and preparing for pen input as well.

But before I add it, I want to do some more cleanups / breaking changes in the existing Application implementations, in particular related to the multi-window support (#124), to not need to make breaking changes and deal with backwards compat in the new Sdl3Application.

[hsdk123]

Cool, looking forward to it coming out!