Add option to select behaviour when splitting a view

Currently there are three possible behaviours:
- duplicate the current view. This is the previous behaviour and remains
  the default one
- have the new view display the start page
- duplicate the current view if it displays a local file and show the
  start page otherwise

The reason for this change is that duplicating a web page requires time
because the page needs to be fetched again and, at least in theory, it
could have side effects.