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
While walking up the tree, only switch the selectedness states
of parents with children. This avoids the case where a parent
has a first subgroup that is hidden -- in which case the
parent ends up with no children, and is unselected even though
it is marked as selected in the config file.
FIXES#864
