QFileDialog static functions (getOpenFileUrl, etc.) could try to stay in the same dir if dir is not given

Related to QTBUG-106385 and https://codereview.qt-project.org/c/qt/qtwebengine/+/431180/2: another reason not to use the static APIs is that if the user tries to open a file, and fails, the user would probably like to try again, and stay in the same directory where it was last time. But if the application provides a directory, it's going to start at that directory; and if it's not given, what's the default directory then? It's not documented. I think if the argument QUrl dir = QUrl() is left as the default-constructed one, then it's Qt's job to be reasonable about this: remember the last directory where the user left it, and go there again the next time.

The overall goal is to have getOpenFileUrl() et al. working well enough that we can use them in our own examples again, and recommend them to users.