removed devtools (to be in separate module)

DevTools should be in separate module. I have created a development module for this purpose (https://github.com/jonathan-reisdorf/mod_development). Feel free to comment or discuss, this is my first PR here, please double check :)
This also contains exposed add/remove methods for the context menu, so that other modules can add/remove entries. Side effect: "Close DevTools" is now only shown when the DevTools are actually opened.