x/y/width/height values went into the calculation for the next popup,
and things got iteratively worse due to padding issues). But the
good news is that this whole subsystem is vastly simpler when
implemented with the new interface.
Move the "tip popup" code from view.nas to a new gui.nas module, and
make it generically useful.
Wire up flap steppings for the 747 as an example of per-aircraft
flaps.
Realtime-based property slewing, to eliminate dependence of trim and
view direction rates on frame rate.