Animated the holder for the headsets.

[SebDieBln]

Clicking on it toggles between "ground" position (horizontal) and "in-flight" position (vertical) as seen in this video.

While it works as intended, the following improvements should be considered:

• Remove the collision between the holder in its vertical position and the windscreen pillar.
• Move the property from door-positions to a new folder, e.g. foldable-positions, that can then hold the properties for other foldable items, e.g. the safety locks for the engine switches.
• Reassign the centerpiece of the holder from the fuselage (interiorwall) to the actual holder (headset_holder), so that the user can click on the centerpiece to start the animation.

[HHS81]

I'll don't delete the branch right now, since there are still issues open as you described.