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.