- new external file for joystick configuration
- designed for a basic three-axis, three-button joystick; others will
require some work
** keyboard.xml
- changed back to using index ('n' attribute) to represent key code
- switched to nesting for modifiers, so that they can all appear in
the same declaration
- bound 'b' to brake, but modified so that the brake is released
as soon as the button is
** preferences.xml
- start mixture at 1.0 (full rich), which is most appropriate for
take-off and landing
- commented out old joystick bindings
- added reference to external joysticks.xml file
