Improve schema.json and add a tool to validate schemas

I've been using this tool on and off locally while working on ERP5 SR, and I think it is polished enough to share it. And also fix the way required properties are defined in schema.json, as these are from an older json schema syntax.

@rafael : Please check the schema.json changes actually match code using it (the UI generation code). I think my changes reflect our original intent, but as it changes a bit which properties are actually documented as required it may require slight adaptations in the code. Hopefully nothing big.

/cc other authors with at least 10 commits in master in 2017: @kazuhiko @alain.takoudjou @yusei @jm @tc @Nicolas