Add sortGroupsFirst option for catalogues

[meh9]

When we have a catalogue group that has both groups and layers in it, at the moment these are sorted alphabetically with groups being interleaved with layers. Several catalogues in our various apps use preserveOrder to manually sort the items in a group so that subgroups appear at the top of the list and layers below them, but each sublist {groups,layers} is still alphabetically sorted. We should add a sortGroupsFirst option that does this automatically.

[meh9]

It has been proposed that we should make this new sort order the default, which I fully support.