[macOS] Updates Import/Export of OPML

- Created OPML UTType
- Added OPML as an importable UTI type
- `allowedFileTypes` replaced with `allowed ContentTypes`
- Moved UTType extension to single shared file
- Build flags updated

Mac/MainWindow/OPML/ImportOPMLWindowController.swift

@@ -85,7 +85,7 @@ import Account
 		panel.allowsMultipleSelection = false
 		panel.canChooseDirectories = false
 		panel.resolvesAliases = true
-		panel.allowedFileTypes = ["opml", "xml"]
+		panel.allowedContentTypes = [.xml, .opml]
 		panel.allowsOtherFileTypes = false
 		
 		panel.beginSheetModal(for: hostWindow!) { modalResult in