This is a function that definitely doesn't always have a non-modifying
behavior across all implementations, so this should be made non-const.
This gets rid of the need to mark data members as mutable to work around
the fact mutating data members needs to occur.
Default implementation will return "yuzu" for any string. GUI clients (or CLI) can implement the Frontend::SoftwareKeyboardApplet class and register an instance to provide functionality.
