Refactor flexible entity querying

Goals are,
 - prepare the support of MongoDB querying
 - allows to add own query part for custom attribute types
 - be able to use the same querying (filtering, sorting) in grids and from product repository

So idea here is to provide a really simple implementation that already encapsulated this concern in the flexible entity bundle to minimize future API breaks,
 - we add a flexible query builder interface (storage agnostic)
 - the current FQB orm implementation is used as service and injected in the flexible repository (I'm not really happy with that but it allows the registrering of new filters / sorters in the FQB)
 - we create some filters and sorters which handle the query part construction

Next steps,
- create MongoDB querying support
- be able to register new filter and sorters for attribute type

Bug fix: no
Feature addition: no
Backwards compatibility break: yes
Unit test passes: yes
Specs passes: yes
Replaced unit test classes removed: yes
Behat scenario passes: yes
Checkstyle issues: no
ChangeLog updated: yes