kcm/qml: Use contextual help when configuring a Printer Group

Remove the always-present text description and use a contextual help button instead. Also, increase the size of the member listview, this generally make the dialog appear more balanced and in line with configuring a normal printer.

This is a low-traffic path and we can remove a couple of items from the dialog. Users configuring groups will understand what they are and if not, the contextual button is there.