use and document QFileOpenEvent in more places

It's documented on its own: https://doc.qt.io/qt-6/qfileopenevent.html and I remember finding out about it once a long time ago (probably when working on QTBUG-34926). But we hardly use it. No widget examples handle this event, but any example or tool that can open files ought to: the widget and Qt Quick text editor examples, the PDF viewer examples, and more. macOS-specific pages like https://doc.qt.io/qt-6/macos.html and https://doc.qt.io/qt-6/macos-issues.html are not reminding users that they need to implement this event handling. I've recently noticed that some things I've written do not handle drag-and-drop or "open with", and had to think for a while: what was that special thing again...

It's unfortunate that this is still a platform-specific feature. We could alternatively come up with some way to avoid users needing to do this macOS-specific thing in every application.

We need a strategy for how Qt Quick desktop applications should easily support this feature, too, without having to remember to do something special in C++. So far it's still handled only in tools/qml/main.cpp ... nearly a decade after we added it there.