(fix) WriteRobot::write() to prefer the Scene of the focused window.

[hastebrot]

This fixes a bug where we previously required to have an extra call to targetWindow("2nd stage").

// given:
interact(() -> window("2nd stage").requestFocus());

// when:
write("foo");

// then:
verifyThat(from(rootNode(window("2nd stage"))).lookup(".text-field"), hasText("foo"));

Note: Probably needs additional considerations regarding to PopupWindow and its subclasses.