WIP: Refactor GuiTest

This is my work-in-progress on extracting "robot" classes from `org.loadui.testfx.GuiTest`. It is a experiment to see if a modular approch is better than the current monolitic approch.

The "robot" classes will mostly have only `void` return values. These classes are (possibly) injected into `GuiTest`. `GuiTest` uses the "robot" classes and provides a fluent api.

ScreenController:

- [x] Extract `press()` and `release()` to `KeyboardRobot`
- [x] Extract `press()` and `release()` to `MouseRobot`
- [ ] Extract `position()`, `move()` and `scroll()`

Others:

- [ ] Extract `TestFxApp`, `setupStage`, `getRootNode`, ...
- [ ] Extract `offset()` and `OffsetTarget`
- [ ] Extract `targetWindow()` and `lastSeenWindow`
- [ ] Extract `find` functionality
- [ ] Extract `point` functionality
- [ ] Extract `captureScreenshot` functionality
- [ ] Extract `waitUntil` and `sleep` functionality
- [ ] Extract `click`, `rightClick`, `doubleClick` functionality
- [ ] Extract `drag` functionality
- [ ] Extract `type` and `push` functionality