Change Qt Quick examples to use natively styled controls

A number of different examples look outdated, and should be updated to look better and more like you would expect from a native GUI application.

Full list: https://intranet.qt.io/pages/viewpage.action?spaceKey=QTRD&title=Graphics+and+UI+Objectives+and+Key+Results

Each example should be changed to use Qt Quick Controls2 (if it makes sense.. for instance the accessibility example should not be changed since its purpose is to show how Qt Quick primitives can be made accessible). By using Qt Quick Controls2 the example should automatically adapt to the desktop theme (and respect e.g. dark theme).

If there is an easy opportunity to make the UI better - feel free to change it. (But this is not always easy - in the examples we prefer readable code over slick UI)