* refactor: Better printing for KeyValue in tests

* Fix parsing of escaped characters in macros

The parsing code was bugged with custom macros like `symbol:\abc` and
`symbol:\\abc`.

This was the case for string keys but not for macros or keys with custom
symbols.

It's replaced with a macro containing one key.

Many kind of KeyValues don't need to be wrapped into a Macro to show a
specific symbol. This is especially useful as Macro keys are not
affected by modifiers.
The parser is changed to have a fast-path when a key value is not a
macro.

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

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.