Add server setting informing plugins / clients to identify a preference for Magnetic OR True paths

[panaaj]

Allow the user to configure which type of path to preference so plugins and clients can use the preferred value type.

Why:
There are a number of paths which offer both True and Magnetic values (e.g. navigation.bearingMagnetic & navigation.bearingTrue).

Example use cases include:

• Course VMG calculation
• Course Bearing to destination calculation
• Displaying wind vectors
• Displaying laylines

Behaviour:
When a selection has been made plugins and clients should use the preferred value type for the relevant operation.
If a value of the preferred type is NOT available the plugin / client should NOT fall back to the other type.

Example:
Preference = Magnetic
navigation.bearingMagnetic = null
navigation.bearingTrue = 0.1546

Operation should handle the null appropriately and NOT fall back to using navigation.bearingTrue.