Distinguish stop names from IDs in the engine

Let the engine know whether it gets a stop name or an ID in requests
by adding a "stopid" data source parameter for stop IDs (previously
IDs were simply given as "stop"). Scripts still get the stop name or ID
in values.stop, but a boolean values.stopIsId was added. For journeys
values.originStopIsId and values.targetStopIsId were added.

Existing providers work like before, but can now use this information.
Done for HAFAS script providers to build better URLs and for GTFS
providers to not unnecessarily try to query the database for the ID.

Use new stop ID parameters in the applet where possible.

Update documentation accordingly.