Custom editor tabs and settings tab

The scheduled publishing feature's designs require a separate settings tab. This adds that tab along with PR #210, which adds functionality to allow users to customise the tab names and the number of tabs.
Do you think this belongs in edit_handlers.py?
Since this is something we expect people to use, I think it would make sense to put this in an easier place to find it such as edit_handlers.py

```from wagtail.wagtailadmin.views.pages import set_page_edit_handler```
vs
```from wagtail.wagtailadmin.edit_handlers import set_page_edit_handler```