Add the ':char' syntax for defining character keys with a different
symbol.
This new kind of keys is used to implement Ctrl combinations in the
Serbian Cyrillic layout without showing latin letters while the Ctrl
modifier is activated.
This allows to add new kinds of keys that need more data without making
KeyValue's footprint bigger for common keys.
This changes the [_symbol] field into [_payload], which holds the same
as the previous field for more common keys but can hold bigger objects
for keys of the new "Complex" kind.
This also adds a complex key: String keys with a symbol different than
the outputted string.
Unit tests are added as the Java language is not helpful in making
robust code.
