New Nasal code, with lots of input binding handlers in controls.nas.

New Nasal code, with lots of input binding handlers in controls.nas.
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.