Bulk Rename is an utility provided by Thunar (the default files manager in XFCE). It's the best renamer I've found so far, but it looks outdated and requires (of course) to have Thunar installed on your system.
This mock-up is basically what the Bulk Rename utility would be if it were a standalone app and written purely in GTK+3.
As a comment I would like to remark how difficult are the Action bars to implement. Surely they are all right for simple editing actions, but I feel that a better widget might fit better in the design. Actually I decided to do the Numbering mode because of its fair simplicity; I'm not sure the other modes could use the Action bar properly... man, I'm not even sure that the mock-up is per se a proper implementation of the Action bars, but whatever.
14.09.30 - Remade the header bar and split the action bar between a toolbar and an action bar containing the Rename action.
14.09.30 - First release.