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.
There was no free bits left to add new modifiers. Instead of increasing
the width of the 'flags' field, refactor the way modifiers are
represented and used.
Modifers are now represented as independent values and stored in the
'code' field. A flag is added to distinguish between modifiers and keys
with a key event.
The most notable change is that modifiers can no longer be or-ed into a
single value but have to be represented as an array.
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.
