EZP-20423 - As a developer, I want to be able to link Form objects to field types

**Work in progress**

The main idea is to extend base field types and make the subclasses implement the new `FormAwareFieldTypeInterface` interface. As such, the field type would be able to return the appropriate `FieldTypeForm` implementation, to be embedded in a bigger form (like content/edit).
The abstract `FieldType` class is designed for *new* 3rd party fieldtypes.

## TODO

- [x] Base classes and interfaces (fieldType + form)
- [x] Basic implementation for `ezstring` (PHP)
- [ ] Template management
- [ ] Basic implementation for `ezstring` (Twig)