Duplicates album fixes

Some spin-offs I made along my way into implementing support for finding duplicate images using a specific album as reference.

Add ref. album to duplicates album tree view. This allows sorting duplicate images per album folder. Also allows both image name and path columns to be resizable by the user.

Remove redundant single physical album signal/slot methods. Replace with alternatives accepting QList<PAlbum*> as parameter.

Add AlbumSelectors::SelectionType. Fixes an issue in which either AlbumSelectors' setAlbumSelected() or setTagSelected() would be called with 'singleSelection' set to false even though the albums list contained only a single item.