Allows to add more keys to the keyboard from a predefined list.
The implementation doesn't use MultiSelectListPreference because it
doesn't seem possible to change the item layout to properly show the
rendered symbols.
The "loc " prefix for predefining a place for an "extra key" was broken
since 31d6a70.
The FLAG_LOCALIZED flag cannot be used anymore, as adding it to any key
would turn it into a different key that wouldn't be recognized by parts
of the code comparing the keys (placing the extra keys).
Add an other layer in KeyboardData to store such informations.
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.
