- added FGBinding::fire(double) - added support for "mod-up" property

- added FGBinding::fire(double) - added support for "mod-up" property
- go back to using index as key code (otherwise, we'll get nasty
  surprises when users try to override bindings)
- XML config file now nests bindings inside modifiers
- allow user to specify whether key is repeatable
- removed hard-coded bindings for 'b', ',', and '.'
- added code from joystick.cxx, with the following major changes:
  - changed from js0, axis0, button0 to js[0], axis[0], button[0], etc.
  - removed factor, offset, etc. since they will be args to command in
    binding
  - allow multiple bindings, as with key mappings
  - allow access to keyboard modifier keys for bindings
  - include infrastructure for modifiers (only FG_MOD_UP is used so far)
  - removed unused 'saturation' property
  - temporarily removed capture property and trim code; it will have to
    be rewritten to be more generic
  - allow modifiers for joystick buttons