#65 Improvements to free-flight function

[chuft-captain]

The acceleration/deceleration function (A and Z keys) currently seems to be referenced to traveling in Universal coordinates only.
This means that if you are traveling above Earths surface in a straight line, your travel path will not be straight relative to the surface of the Earth below you. (A coriolis effect will be observed)
[Also, strange things happen in SYNC mode -- when pressing the A key, the motion appears to shoot off to one side rather than straight ahead. It only seems to work correctly in FOLLOW mode]

What I was thinking is that now that we have all these wonderful reference frame capabilities, would it be possible to add a new capability to reference the motion created by the A key to the bodyframe of the selected object. -- This would allow genuine straight line travel relative to a point on a planetary or other surface.

This might also fix the problem in SYNC mode.

If there are no conflicting issues, the cleanest implementation might be to default to the reference frame of the currently selected object.. and allow travel on both FOLLOW mode as well as SYNC mode. If no object is selected then revert to the status-quo (Universal frame)