Migrate away from deprecated methods for setting the standard application on macOS

The methods for checking if qBittorrent is set as the standard application for opening torrent files and magnet links and setting qBittorrent as the standard application for those is using deprecated methods now. For example `LSCopyDefaultHandlerForURLScheme` and `kUTTagClassFilenameExtension`.
The new methods have been moved to `macutilities.mm` because in `os.cpp` cocoa couldn't be imported.

PR #23059.

src/base/utils/os.h

@@ -48,13 +48,6 @@ namespace Utils::OS
 {
     void shutdownComputer(const ShutdownDialogAction &action);
 
-#ifdef Q_OS_MACOS
-    bool isTorrentFileAssocSet();
-    void setTorrentFileAssoc();
-    bool isMagnetLinkAssocSet();
-    void setMagnetLinkAssoc();
-#endif // Q_OS_MACOS
-
 #ifdef Q_OS_WIN
     Path windowsSystemPath();