Groups with fewer than two items serve no purpose. All
user-facing group creation paths now require at least two
items before creating a group. The Group menu action is
greyed out when fewer than two items are selected.
When items are removed from a group via RemoveFromGroup,
pruneExistingGroups, or the netlist updater, the group is
dissolved if it drops below two members. Importers are left
unchanged since they reflect external file structure.
This also removes the GROUP/UNGROUP-specific undo actions.
This also fixes a bunch of undo bugs when duplicating
group members, creating pins, etc.
This also fixes some undo bugs when dividing wires etc.
This also fixes some bugs with new sch items not
being created within an entered group.
Seth Hillbrand