LP#1710401: Z39.50 should notice overlay target change

LP#1710401: Z39.50 should notice overlay target change

Currently, we record the overlay target at UI startup and use that going
forward until the interface is reloaded.  This commit inspects the local
storage version of the target for changes and offers the user the chance to
proceed with the new target or cancel the action.  If the target has been
unset, the user is given the option of proceeding with the load-time target.

Signed-off-by: Mike Rylander <mrylander@gmail.com>
Signed-off-by: Chris Sharp <csharp@georgialibraries.org>