Rewrite screencapture example in QML

We are now in the process of supporting screen capture on Android (see relation chain: https://codereview.qt-project.org/c/qt/qtmultimedia/+/609291/34)

Our screencapture example is currently written in Qt Widgets, which works very poorly on mobile. This makes it both hard to test the Android screen capture functionality during maintenance, and also means our customers won't have a good example to use when trying out our screen capture functionality on mobile.

Doing this can also help us identify parts of our APIs that have lacking QML functionality (missing signals, undocumented properties etc.)

We should rewrite the screencapture example to use QML in order to showcase the functionality as best possible.