[FEATURE] added sections and additional informations to xml-template to generate better forms

added sections and additional informations to xml-template to generate better forms:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] adapted xsd template
- [x] refactoring template reader
- [x] adding sections to template reader
- [x] adapted cache class generation
- [x] refactoring templates (https://github.com/sulu-cmf/sulu-standard/pull/88)
- [x] changed form generation (https://github.com/sulu-cmf/SuluContentBundle/pull/134)


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/300-webspaces/DET-301-template-architecture.md#sections and updated example
@wachterjohannes Is this really a good idea? I would use a variable like that only in a strategy.