Add "macro" keys that behave as if a sequence of keys is typed.

Macro can be added to custom layouts or through the "Add keys to the
keyboard option". The syntax is:

    symbol:key1,key2,..

The symbol cannot contain a : character. 'key1', 'key2', etc.. are:

  - 'String with \' escaping'
    The key will generate the specified string.
  - keyevent:123
    The key will send a keyevent.
  - The name of any special key

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.

Incompatible APIs were used in the custom layouts and the extra keys
options.

Add @TargetApi annotations to help catch similar issues in the future
with the help of 'gradle lint'.