WIP: ZODB Components: Likewise Document, add Mixin (erp5.component.mixin) and Interface (erp5.component.interface).

* One Mixin/Interface class per ZODB Component.
  => Already the case for FS Mixin, not for Interfaces.
* ZODB Components module name ('reference' property) and class name:
  + Mixin: FooMixin.
  + Interface: IFoo.

Rationale:
  + Avoid current FS hacks: registry (Mixins, mixin_class_registry) or import
    all classes explicitly in __init__.py (Products.ERP5Type.interfaces).
  + Consistent naming.
  + Consistent with ZODB Documents Components.

TODO BEFORE MERGING:
  + Add Unit Tests.