Improve .torrent associate handling

* Use correct pointer type in NSIS scripts
* Only remove qbt specific registry keys when uninstalling or disassociating
* Set .torrent Content Type when associating .torrent format
* Move ".torrent association" functions to Utils::OS class

PR #19709.

src/base/utils/misc.cpp

@@ -651,4 +651,4 @@ Path Utils::Misc::windowsSystemPath()
     }();
     return path;
 }
-#endif
+#endif // Q_OS_WIN