iOS: Add native directory picker

QFileDialog::getOpenFileName() uses a native dialog, but QFileDialog::getExistingDirectory() doesn't.

iOS 13, which is the minimum version supported by Qt 6.3 (https://doc.qt.io/qt-6/ios.html ), introduced the ability to pick a directory: https://developer.apple.com/documentation/uikit/view_controllers/providing_access_to_directories