/sim/input/selected/engine[n] properties. Also, new properties get
empty string values (which are boolean true in Nasal, maybe that
should be changed?), so test for numeric truth.
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.
