Continued/broken/overcomplicated attempt at modeling flexible subgroups as a separate thing,
when it seems better instead to rollback some(?) things from a few prior commits
and just use entities which contain groups, and put the rest of the smarts &
convenience in the UI.  Should go much faster since less code will have to
be written.
Revert "Continued/broken/overcomplicated attempt at modeling flexible subgroups as a separate thing,"

This reverts commit 0072e950bd0e475e4a52accad58f904290723881.
Revert "Rename table subgroup to groupsinagroup (prep for next work)."

This reverts commit 0b79e05bab39cdff3ad4927c23656e964522b54d.
Revert *parts of*:  "Bugfixes, and make new entry in group be sorted immediately after the selected one."

This reverts PART OF commit ad345e0e0f0bc97b61b89d5e22142031f63827f4.
Clean up the subgroup stuff to prep & do a simpler approach.
wip