Allow pool registration depending on activated modules
For now, we have the option to register mixin for a model that is defined in an extras_depend. For this case, we just ignore it silently.
But this is not enough to support all cases. Sometimes we want to be able to add a field on a model only if a module is activated because the field needs this module (ex: the target of a many2one).
So the proposal is to allow to register classes in the pool under a depends condition.
This way, we could remove the silent ignore of mixin registered without proper class. This will also make the registration process more explicit about the dependency.