Update of outdated Qt Examples

Some of the examples, that come with the precompiled Qt-versions - or with the source code - haven't been touched in years. As a result, they promote questionable methods, e.g. the use of QCoreApplication::ProzessEvents(), or are structured in a way that has fallen out of form.

For example:

https://doc.qt.io/qt-5/qtwidgets-dialogs-findfiles-example.html

https://doc.qt.io/qt-5/qtwidgets-itemviews-pixelator-example.html

https://doc.qt.io/qt-5/qtwidgets-painting-fontsampler-example.html

These should be revised with the new features Qt, 5.9 as the latest supported LTS version, and the c++ standard now offers.