support graph import of JSON v3 data files

[dneuge]

JSON v3 format is missing some information which was previously available, such as flight plan revision numbers. Graph import currently relies on that information, so it needs to be adapted in order to fully support JSON data files.

[dneuge]

Differences observed so far:

• ATC stations no longer have coordinates
• observing pilots appear as ATC_CONNECTED
• "last updated" timestamp (now alias getControllerMessageLastUpdated()) has unknown meaning:
  • no longer reflects time of controller message update
  • available for prefiles and pilots as well
  • appears to be related to data file update (age of data snapshot?), not content updates
• flight plan revision has been removed
• "actual departure time" has been removed (or merged into deptime?)
• effective client type = raw client type?
• protocol revision has been removed

[dneuge]

Check if compatibility with legacy data files can be maintained using one solution for legacy and JSON formats which might actually solve #1 and #12 as a by-product.

[dneuge]

Flight plan revision has been readded without notice: #19

Graph import could most-likely work as before without changes.