Revamp Dynamic Scene example

"Dynamic QML Object Creation from JavaScript" is linked from quite a few places, and –whether we like it or not– people tend to actually use that functionality. We should

• add a (back-)reference to dynamically creating objects in a more declarative way (model/view classes)
• Modernize the CMakeListst, QML and JS code
• [less urgent] Check whether the visuals of the example could be improved

Support for QDS is obviously antithetical for this example