Let's make SelectionBehavior a view concept where views can either
select individual items (row, index) or whole rows. Maybe some day
we'll do whole columns, but I don't think we need that now.
Views now have a cursor index (retrievable via cursor_index()) which
is separate from the selection.
Until now, we've been using the first entry in the selection as
"the cursor", which gets messy whenever you want to select more than
one index in the model.
When setting the cursor, the selection is implicitly updated as well
to maintain the old behavior (for the most part.)
Going forward, this will make it much easier to implement things like
shift-select (extend selection from cursor) and such. :^)
A view can now be told to move its cursor in one of multiple directions
as specified by the CursorMovement enum.
View subclasses can override move_cursor(CursorMovement) to implement
their own cursor behavior. By default, AbstractView::move_cursor() is
a no-op.
This patch improves code sharing between TableView and TreeView. :^)
