tab_switcher: Add support for tab switcher in assistant panel (#15475)

Additionally, I've generalized the implementation of tab switcher so
that - instead of explicitly listing panels it supports (at the time of
writing it was just the terminal panel and nothing else), it now relies
on Panel::pane trait method. As long as that's implemented, you get a
tab switcher support for free.

Release Notes:

- Added support for tab switcher in Assistant panel.