Consider allowing multiple selection on end node groupings

[zspitz]

The current design is that only one end node grouping can be selected at a time.

Would it be useful from a UX perspective to allow multiple groupings to be selected at once?

The logic in the UpdateSelection command would be different: for each grouping, if group is selected, select child nodes; if group is unselected, unselect child nodes.

The current logic for when a grouping is selected, is to deselect all other groups.