The meaning of the public fields of KeyValue was quite complicated and
not handled consistently accross the app.
Make these fields private and add a more abstract API on top.
The meaning of these fields changed recently and it wasn't an easy
change. I plan on making more changes in the future.
KeyValue defines an ADT and some of its values, it now contains public
final fields and no internal logic.
KeyModifier handles modifiers and accents and creates new instances of
KeyValue when needed.
This operation is now properly cached.
