Add GTFS importer and accessor

- The GeneralTransitFeedImporter imports GTFS feeds and writes the data
with little changes into an SQLite database, eg. using integer IDs,
insead of strings. Importing the data may take some time to complete,
the data engine should report progress here (TODO)
- Add reading of "<feedUrl>" tags to AccessorInfoXmlReader, contains
the URL to the GTFS feed for GTFS accessors. Currently that URL is
stored as DepartureRawUrl, but with new getter/setter functions
- TimetableAccessorGeneralTransitFeed uses the database to get
timetable information. If the database does not exist, it downloads
and imports the GTFS feed (new <feedUrl> tag).
It overwrites TimetableAccessor::requestXXX() functions instead of
::parseXXX() to directly return data from the database, no need to
wait for a download to complete