FlightGear - Flight Simulator FGAddon

- Add the Protocol/headtracker.xml file in the addon, so that one does
not need to download/install it separately.

- Add a menu and configuration dialog. It allows to disable
headtracking, and to selectively disable translation axes, roll axis,
and headtracking in external views.
(default unchanged: all axes enabled, disabled for external views.
However internal views are determined with /sim/view[i]/internal,
instead of restricting to cockpit view.)

- Handling view rotation offsets:
The best example is the right-facing flight engineer views (e.g. 707
or Tu-154). With the old addon, moving the head forward would move the
view in the aircraft forward direction, which is to the left of the
flight engineer. As a result, translation controls are shifted 90°
from what one would expect.
(and space shuttle rear-facing views are even worse!)

This changes it to move the viewpoint forward in the view direction.
It also improves the way the headtracking rotation is composed with
the view rotation offsets.

- Minor readme/metadata changes, mostly to mention Opentrack which uses
the same protocol as Facetracknoir.