todo list updates
Revert "Work in progress while reviewing last commit/fixes, until realized"

This reverts commit 06de3fea433919bd10eda5c66f3fa58e9dfd0944.
Revert "Allow nesting groups in more than one containing group."

This reverts commit 2ea4aca681a0df83d44845e7d4fe373993c8651b.
README updates.
Clarify on menus that existing entities (therefore subgroups) can be added to groups.
Very basic 'search for an entity' feature.
bugfixes
Allow nesting groups in more than one containing group.
(Realized that while it is not needed to add an existing subgroup (actually
its containing entity) to a group, I do need this code after all, in order
to add to an entity a relation to an existing group.
Allow doing a search when adding relation to an existing group to an entity.
Bugfixes for import when not creating a new group first, but
putting all top-level items from the imported file as new entries
directly.
Allow import to existing groups (previously just was on entities), & bugfixes.
Fix the transaction management so that importing can again roll back if desired.
Make imports repeatable w/o crash on error, and classes mixed by default.
Little fixes (naming only).
Bugfix.