Oberon System 3 compatible with the Oberon+ compiler and IDE

This is a version of the Oberon System 3 (also known as ETH Oberon), compatible with the Oberon+ compiler, IDE and runtimes and the OBX Platform Abstraction Layer (PAL), and thus truly cross-platform (runs on all platforms where LeanQt is available).

The migration is still work in progress, but sufficiently complete and stable to explore the platform. The latest commit is tested on both the Mono CLI and as a native executable built with the generated C code.

I have to admit that while I'm aware of the Oberon System, I know far too little about it to make any meaningful statements here.