Provide an up to date xml schema for all xml formats

Hi,

I'm now at a phase where I can start writing some custom map conversion tools. I was looking at the bundled Java library but unfortunately that one is really out of date. Then I looked at the schema's in de doc directory to just make my own parser. But they are also out of date.

Ideally the schema should serve as the single source of truth for:

• Tiled's own xml handling code
• Any parser library (including the bundled one)
• Documentation

This also forces you to have an up to date XSD at all times.

For now I think I can use TiledReader. But I think it would be really useful to provide up to date xml schemas for writing tooling around tiled.