[QtSCXML] Cpp datamodel usage demonstration

The mediaplayer example, which is an example streamlining drop candidate in 6.5, demonstrated one unique aspect of statemachines; how to use the Cpp datamodel.

This is an important aspect to show and this task is about introducing that demonstration elsewhere. The two primary possibilities are:

• Extend an existing example application to demonstrate it
• Create a documentation code snippet to demonstrate it