* Don't require a workspace to be loaded in order to render the group
header menu.
* When adding or removing root folders, do it to *every* workspace in
the group.
* When activating a thread, never open a different window, and never
open it in a workspace that's part of a different groupw with a superset
of the thread's worktrees. Find or create a workspace with the exact
right group of root folders.

Release Notes:

- N/A