Added keybindings for selecting a current engine, for changing magneto

Added keybindings for selecting a current engine, for changing magneto
position, and for engaging the starter.  Here's how it works:

1. To select an engine, use '!' (shift-1) for engine 1, '@' (shift-2)
   for engine 2, '#' (shift-3) for engine 3, '$' (shift-4) for engine
   4, or '~' to select all engines.  These bindings set the bool
   properties /sim/input/selected/engine[0],
   /sim/input/selected/engine[1], etc.

2. To decrease the magneto for the selected engine(s), use '{'.

3. To increase the magneto for the selected engine(s), use '}'.

4. To engage the starter for the selected engine(s), use SPACE.

Eventually, I'll extend this so that the throttle uses the selected
engine as well, and will perhaps even support engine selection in the
joystick bindings.  Right now, only engine 1 (engines[0]) is selected
by default.