Update of JSON schema definitions

[edgarmueller]

Fixes issues #856 #938, and #998.
Make JsonSchema a union type of JsonSchema4 and newly introduced JsonSchema7. Update validator accordingly to use option auto which also solves issue #998. Some things to note:

• The schema generator needs to known about the generated version, hence it keeps using v4 for now
• The editor is due its usage of id specific to v4

Further improvements (e.g. supporting additional keywords) and refactoring (editor component) should be handled in separate issues, such that we can close #856 .

[coveralls]

Coverage decreased (-0.3%) to 87.69% when pulling df362f9 on edgarmueller:topic/update-schema-version into 24fdf60 on eclipsesource:master.