A track group's name, membership and Edit/Mix flags could only be set
when it was created: nothing in the engine or the UI could change them
afterwards, so a typo in a group name was permanent.
Add track.group_modify {group: id|name, name?, tracks?, edit?, mix?}.
Omitted fields keep their value. `tracks` replaces membership only when
it is an array, so a mistyped param cannot silently swap the members for
whatever happens to be selected; it dedupes, rejects an empty group, and
rejects an empty or duplicate name, since find_group resolves by name.
The group's letter is left alone: it is the stable id in the Groups list.
Dialog::Group gains an optional group id. None creates, as before; Some
pre-fills from the group and dispatches track.group_modify, so one dialog
serves both. Reached from Modify Group... in the Groups list context menu.
OK is now disabled while no track is checked, since both commands reject
an empty group and the old OK could only raise an error.
The command has no menu path, so it adds no menu leaf and the parity
checklist is unchanged.
Audiosyncrasy