Integrate tab bar with sidebar on iPad

[Sjmarf]

IMO this is objectively better than what we have currently, we just need to work out how to do it well. Ideally we'd keep state when switching between tabs like the current tab bar does, which could be tricky.

https://developer.apple.com/documentation/uikit/elevating-your-ipad-app-with-a-tab-bar-and-sidebar

Requested in #251.