[Windows registry] Allow to clean all swift registry entries

* for uninstallation
* few users reported a hanging (white screen) swift issue, solved by "cleaning the registry"

see https://discordapp.com/channels/539048679160676382/539925070550794240/664557031784448010

src/swiftlauncher/swiftlauncher.h

@@ -216,6 +216,9 @@ private:
     //! Run in installer mode
     void installerMode();
 
+    //! Clear Windows registry
+    void clearWindowsRegistry();
+
     //! Command line
     static QString toCmdLine(const QString &exe, const QStringList &exeArgs);
 };