Allow to customize the model form directly on the model declaration.

Hello,

I needed this small modification to customize the `model_factory()` in `get_form_class()` of the `BaseAdapterField` in a simple manner.
This allows me to exclude model fields that are not meant to be handled via forms, or to use only a subset of `fields` in `inplaceedit`.
For example I have some JSON fields that are for internal use only, that are `blank=True`, but make `inplaceedit` crash because their `clean()` seems not clever enough (or perhaps it's me, but whatever).

A foreseable extension of this could be to add something like:

```
class MyModel(…):
    INPLACEEDIT_FORM_CLASS = 'myproject.myapp.forms.MyModelForm'
```
And make the `get_form_class()` use this class attribute if present. But I didn't need this level of customization yet.

Best regards,

Olivier