QSettings: Provide a means to allow fallback writing (similar to QSaveFile)

QSettings: Provide a means to allow fallback writing (similar to QSaveFile). It is possible that the settings file is in a directory that does not have permissions to create new files. Therefore things like the lock file and the QSaveFile usage internally will cause it to fail to write to the settings file although it could actually do so directly.

Therefore adding similar API to what QSaveFile (directWriteFallback) would be ideal, granted with the warning that writing has to be adequately protected in this case meaning it is up to the user to ensure concurrent writes are not happening.