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.
New Node.setValues() method which sets whole property trees from Nasal
data.
A view.nas module, which takes over handling of the X/x zoom keys. It
clamps the FOV to a dynamically calculated maximum corresponding to
typical human visual accuity, and pops up a pretty dialog informing
you of the new FOV.
