Re-create or remove Qt Quick photoviewer example

Qt Quick photoviewer example (https://doc.qt.io/qt-6/qtdoc-demos-photoviewer-example.html) is rather outdated and could be re-created to better highlight the current best practices.

Showing images is in principle a good example and this can be simple enough to be an example.

It would be possible to significantly refactor the existing example or completely replace it with another photoviewer example. Alternatively, remove the photoviewer and polish photosurface (https://doc.qt.io/qt-6/qtdoc-demos-photosurface-example.html) example for Qt 6 best practices and to work with Design Studio.

The example should follow Qt 6 best practices, including current Qt Quick Controls and work with both Qt Creator as well as Qt Design Studio.