SDL_EnumeratePropertiesCallback should return SDL_EnumerationResult

[icculus]

This is SDL4 API-change stuff.

SDL_EnumeratePropertiesCallback returns void. It should return SDL_EnumerationResult, so an app can stop doing work once it found the thing it was looking for (and also dictate if SDL_EnumerateProperties should return success or failure in the stopped-early case).

Along with this, SDL_EnumerationResult should move out of SDL_filesystem.h and into a more generic location.

[icculus]

There are maybe other callbacks that would also benefit from this sort of change, but I haven't looked around for them at the moment.