refactor: add explicit this-> where possible

I have knowingly skipped some files/portions of files where this would
create merge conflicts for other open PRs.

src/singletons/Toasts.cpp

@@ -462,7 +462,7 @@ AvatarDownloader::AvatarDownloader(const QString &avatarURL,
         {
             this->file_.close();
         }
-        downloadComplete();
+        this->downloadComplete();
         this->deleteLater();
     });
 }