MobileForm: Cleanup combobox and use more appropriate naming

This cleanups some of the doc, and also renames the delegate properties to be more in line with the displayMode property.

The only usage of those properties I could find are at https://invent.kde.org/pim/itinerary/-/blob/master/src/app/CountryComboBoxDelegate.qml

I also removed the scrollview for in the dialog because one is already supplied by Kirigami.Dialog (otherwise we end up with a scrollview in a scrollview).