PIVOT-948:  Allow selected key bindings to completely decide for themselves
what to do with an unselected value in ListView, TableView, ListButton and
Spinner.  Currently the item to be stored is always set to null when nothing
is selected, which is correct, except that the binding "get" method should
be allowed to decide what the item value should be in all cases (as is done
in "load").



git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1593659 13f79535-47bb-0310-9956-ffa450edef68