Most input and GUI files are now in a common ui/ directory. The platform/
directory is no longer, as everythyng was moved to application/ or ui/.

In particular, get rid of the enum magic; we can use map<string,int> if we
really need some named enums. Which we usually don’t.

I was arguing that Enter and other keys should be part of the text
input API, but it’s hard to decide what Tab actually does. I prefer
to give more freedom to the client application.

We were no longer calling Render() on primitives with a null source.

We now just look for SDL2/SDL.h (the regular location) and SDL.h (when
the user provides their own location for this header).

The scene now owns its renderer, so there is no need to keep a list
of existing renderers and to call Renderer::Get().

This was not a very smart class. We replace it with a bidirectional map.