multishell - shake out some initial multishell-list glitches
- (Offer to) delete shell buffer, if present, when deleting an entry.
Rely on the default active-shell deletion prompting for confirmation,
supplementing with our own confirmation for inactive shell buffers.
- Set recency (numeric rank) as initial sort field, get closer to
stable sort - still needs work.
- Recompute list on any operation that affects it, and try to
preserve stability. Also needs more work.