Improve content file/folder names handling

Move files/folders renaming functions to core classes.
Query file/folder for renaming by its current path.
Add ability to rename content folders from WebAPI/WebUI.

src/webui/api/torrentscontroller.h

@@ -84,4 +84,5 @@ private slots:
     void toggleSequentialDownloadAction();
     void toggleFirstLastPiecePrioAction();
     void renameFileAction();
+    void renameFolderAction();
 };