Root is always selected, can't be unselected, and has its own explicit
constructor and name. This resolves issue reported where unchecking
all *visible* groups caused the root to be unchecked, after which
hidden-but-still-selected subgroups were not installed.
Reported by crazy@
If a subgroup is hidden, then it should be considered
selected if its parent is selected or partially-selected.
If the parent group is totally unselected, then the hidden
subgroup shouldn't be installed either. This allows putting
required-packages into a group, without cluttering the
interface.
FIXES#864
