1. Fixed FGInput bug preventing same key from being bound separately

1. Fixed FGInput bug preventing same key from being bound separately
with different modifiers (the format of the key bindings has changed
again slightly, adding a <code>..</code> element; see keyboard.xml for
details).

2. Modified FGInput to try default modifiers for ctrl, shift, and alt
when the initial bindings fail (i.e. you don't have to specify
mod-shift explicitly for upper-case 'P' any more).

3. Fixed problems with properties and panel reloading reported by
Martin Dressler and John Check.

4. Updated preferences.xml to get rid of obsolete references to
/controls/brakes/left and /controls/brakes/right (as reported by John
Check).

There were also two non-bugs reported by John Check:

(a) Differential braking doesn't work (John was using the obsolete
properties from #4 above).

(b) Duplicates show up in the property tree using the telnet interface
(the property tree doesn't show indices, and John was seeing separate
entries for each possible engine, etc.).