Provide inline editing / a lightbox for "foreign key" plugins

[vdboor]

When a plugin is only a foreign key to an existing model, it would be nice to open that model somewhere inline (e.g. in a lightbox) instead of only offering the pulldown field to select a model.

[gannetson]

For reverse fk models you could use something like this: https://github.com/onepercentclub/django-fluent-contents/commit/f67cb5c8ed401a85f6c2b03db54888aa3497bebf

I can make it into a pull request (with documentation and all) if you like it.

[vdboor]

Hi @gannetson, that is an amazing improvement! Right now there are many changes in the develop branch, including the way content items are created (as preparation for nested items). If you can create a pull request upon develop, that is more then welcome!