R1:189910d6157c

Make TabGroups a standalone UI element

Summary:
Ref T10628. Currently, tabs are part of ObjectBoxes. However, the code is a bit of a mess and I want to use them in some other contexts, notably the "prose diff" dialog to show "old raw, new raw, diff".

Pull them out, and update Files to use the new stuff. My plan is:

- Update all callsites to this stuff.
- Remove the builtin-in ObjectBox integration to simplify ObjectBox a bit.
- Move forward with T10628.

This is pretty straightforward. A couple of…