Better layout for each language's distribution. Now including all languages in the distribution of strategoxt. And generating a tarball with all languages and binary wrappers for many transformations

This is a squash of 7 commits:

Make GPP distribution more standard

Make ATerms distribution

Make Sdf2-front distribution

Make Stratego-front distribution

Make RTG distribution

Update build files to handle the new distribution layouts

Generate a tarball as a distribution including all languages