Also simplifies groups so that other areas of code that have to know
about them at least don't have to know as much. One of the simplifications
is to not worry so much about empty groups until save time; others are in
the access logic to parent groups.
Also simplifies user model slightly by removing Merge and Flatten
(which are just ungroup/group and ungroup/ungroup/.../group).
Also allows multiple groups to have the same name. This is useful when
using groups for a classification system.
Fixes https://gitlab.com/kicad/code/kicad/issues/5788
Jeff Young
Ian McInerney
Qbort
Joshua Redstone