xmltv: Add option to save epgdb after xmltv import.

xmltv: Add option to save epgdb after xmltv import.

The "periodic save database" means that for xmltv you can
import your daily listings, crash, restart, and not have xmltv
data since the periodic epgdb timer has not elapsed.

So, add an option so the user can save the database after the
import has completed, assuming changes occurred.

This save is delayed by a couple of minutes in case the user
is importing from several different xmltv guides, in which case
the save occurs after the last import.