Add support for earlier/later journeys to the applet

- Show "Get Earlier/Later Journeys" actions in the context menu
and as TimetableListItem in the journey list
- New graphics widget TimetableListItem, used to place actions
into a PublicTransportGraphicsWidget, ie. the earlier/later
journeys actions
- Add Flags to JourneyTimetableWidget to enable/disable
the ealier/later journey actions, when the provider does not
support the ProvidesMoreJourneys feature, these actions should
be disabled because they won't work
- Use the new service operations "requestEarlierItems" and
"requestLaterItems" to do the work