Closes #ISSUE

Removes a few eager container clones and iterations.

Added a todo to `get_prev_tab_group_window` and
`get_next_tab_group_window`. They seem to use `HashMap::keys()` for
choosing the previous tab group, however `.keys()` returns an arbitrary
order, so I'm not sure if previous actually means anything here. Conrad
seems to have worked on this part previously, maybe he has some
insights. That can possibly be a follow-up PR, but I'd be willing to
work on it here as well since the other changes are so simple.

Release Notes:

- N/A