FolderListModel needs to have API review, fix some API warts and be a maintained API instead of Qt.labs

Some obvious warts are

• it uses QUrl for folder, rootFolder and parentFolder properties but the paths returned from the model are string paths, not QUrls.
• rootFolder and parentFolder properties are undocumented
• folder property is a QUrl but the doc says it's a string

The FileDialog provides URLs, and since the QML implementation uses FolderListModel, it needs to make the conversions itself. So that will need to be fixed at the same time.