all Q_PROPERTY declarations in examples and documentation should include FINAL

We don't have a proper migration path for the default attributes of Q_PROPERTY, yet, but FINAL should definitely be the default. Overriding properties is rarely done on purpose and often causes confusion. Also, the Qt Quick Compilers have a hard time dealing with potentially overridden properties.

What we can do already today is push people to use FINAL in all their properties. One way to do so is add it to all places that may get copy-pasted into user code.