Add 'Unattached' filter for galleries.

props pento.
fixes #29329.

git-svn-id: https://develop.svn.wordpress.org/trunk@29722 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/js/media-views.js

@@ -5618,6 +5618,16 @@
 						order:   'ASC'
 					},
 					priority: 20
+				},
+
+				unattached: {
+					text:  l10n.unattached,
+					props: {
+						uploadedTo: 0,
+						orderby: 'menuOrder',
+						order:   'ASC'
+					},
+					priority: 50
 				}
 			};
 		}