tui: fix selection bugs after deleting a connection
nmt_newt_listbox_clear() did not reset active and active_key, which in the case of NmtEditConnectionList meant that after the connection list was rebuilt, the selection would appear to be in the same place, but active_key would still point to the connection that used to be in that row, rather than the one currently in that row, so if you immediately hit Edit or Delete, you'd get unexpected results. (It also meant that it was possible for the selection to land on a header row instead of a connection row.) This was particularly bad in the case of the Delete button, since active_key would be left pointing to a freed NMConnection in that case. Fix NmtNewtListbox, and then add code to NmtEditConnectionList to preserve the selection itself when rebuilding the list.