Warn the user before auto-generating large proxy files
After creating a new (empty) project and drag'n'droping a video file in an unsupported format, Pitivi automatically generates a proxy file at the same resolution than the original file. However, in some case (as reported here), the proxy file could end up being much larger than the original file and fill up disk space dramatically if the user drag'n'drops several videos at once. Should the user be warned about that risk and be offered to disable auto-generating?