Support for page variants

[robmoorman]

Deliverable for the sprint can be a RFC.

Currently page variants are shown in the same list in the page explorer/list. It would be nice to exclude these page variants and edit them via an e.g. extra button beneath the page title (just like 'actions').

The solution should easily to be used by other third-party apps; like wagtailtrans and wagtail-experiments.

[mvantellingen]

I think we indeed need to come up with a generic solution. Currently we only have Page -> PageRevision but perhaps we should create something like PageFacade -> Page -> PageRevision.

This allows the PageFacade to select which page needs to be used for the specific request