Define IReceiver.
Define Index object which indexes objects according to interface.

The intended use of the Index is to allow the display of objects in
spatial views (such as the map and spectrum views) more generically than
the current implementations (ReceiverMarks and Map addModeLayer).
Neither has been updated to use it yet.
Put index into widget context.
Use index to add hairline from hamlib to ReceiverMarks.
Client test suite notices syntax errors.