feat(Robot): Support for Unicode strings in WriteRobot.

This PR adds support for Unicode strings in WriteRobot.

It uses `JavafxRobotAdapter` to fire `KeyEvent`s with the string. This greatly simplifies keyboard input with different keyboard layouts. One downside is, that `JavafxRobotAdapter` requires a `Scene` as parameter. This `Scene` is retrieved via `WindowFinder#target()`.

Examples:
~~~java
fx.write("aeAEäéÄÉčČ.:12");
fx.write("ゼルダの伝説");
fx.target(node.getScene()).write("foobar");
~~~