#844: Refactor Builder component

With this PR we implement the suggested refactoring or issue #844.

- [x]  Refactor all build* steps to assemblers
- [x]  Implement a 'Buildstep' in a generic Builder that assembles, filters and then validates 
- [x]  Extract filtering to a filtering manager using Zend\Filter
- [x]  Extract validation to a Validator using Symfony Validator

Open considerations:

- Move Assemblers to Core plugin as they create a dependency between Reflection and Descriptors
- Move filters to Core plugin as they are specific to phpDocumentor
- Move validators to Core plugin as they are specific to phpDocumentor