Modernize code used in examples

Go over all examples and modernize/check the code.

The general rules/guidelines for all Qt module examples should be adhered to & can be found under https://wiki.qt.io/Qt6/Example-Guideline.

The QML code should have no warnings from qmllint.
The CMake API should use qt_add_qml_module were it makes sense.

Start using other new features were it makes sense.