Add a placeholder for showing VI version on status page of Enabler.

See #148.
Limit the VehicleService to a single VI for a much cleaner API.

Fixed #166.
Merge pull request #166 from openxc/166-only-1-vi

Limit VehicleService to having only one VI enabled at a time
Play with timing on another test case (better solution, anyone?).
Remove unnecessary layout wrappers in diagnostic request view.
Merge remote-tracking branch 'origin/display-vi-version' into 167-vi-connected-callback
Disable existing VI when switching to trace data source.
Add a callback for when the VI connection status changes.

This also adds placholders for each VI to return the version.