Remove generated-from-json-schema java files from the repository

(They are regenerated on each recompile)
Invoke own version of jsonschema2pojo, using LibGDX Array

Use es.e-ucm:jsonschema2pojo-maven-plugin, which allows
arrayDefinition and arrayImplementation classes to be specified

Updated all accesses to the schema to refer to Arrays instead
of List or ArrayList. There is mostly a 1-to-1 translation.

Also changed the engine's schema pom, because LibGDX now has to be
available in the schema.
Add an "Indexed" annotation, showcase use from conversation.json

This annotation can be used to improve indexing and full-text search
limiting it to where it is really most useful.
Updated .gitignore to ignore generated schema classes
Merge pull request #656 from manuel-freire/update-jsonschema2pojo-generation

Update jsonschema2pojo generation