Add missing QTreeWidget::dropEvent to d'n'd atempt occuring in same location.

Remove 'Overwrite attempt' dialog as obsolete. Now moving item, e.g. from folder to same folder, will just make it the last child of the folder.

src/rss/rss_imp.h

@@ -79,7 +79,6 @@ private slots:
   void askNewFolder();
   void saveFoldersOpenState();
   void loadFoldersOpenState();
-  void displayOverwriteError(const QString &filename);
   void on_actionManage_cookies_triggered();
   void on_settingsButton_clicked();
   void on_rssDownloaderBtn_clicked();