Added ops for Selection; Deleted SimpleListAdapter.

Selection now uses OpObservableList, allowing more refined responses
when selection changes. This follows a similar pattern as seen in
ListAdapter for data changes, where an ops handler method makes calls
specific to each possible change op.

SimpleListAdapter, whose original purpose was to provide an easy to use
alternative, has been deleted, because it actually requires more
complicated handling. And, with a new ListItemLabel and dedicated
SelectableStringItem, ListAdapter can be set with a list of strings,
covering the simple case.