Allow users to remove folders from Add/Rescan Files dialog

[Lowlander]

Users should be able to delete folders from the main Add/Rescan Files dialog. It took me a minute to figure out that you need to open Choose Folders to deselect folders for scanning.

The Delete key should do the trick on a selected folder (row) and/or setting the drop-down to Don't Scan (would that remove it next time the dialog is opened??).