I have now created an extension that adds the requested button:

![enter image description here][1]

The extension is available on [GitHub](https://github.com/oliversalzburg/se-kbd-button) and should work as:

- [Userscript](https://github.com/oliversalzburg/se-kbd-button/blob/master/src/common/se-kbd-button.user.js)
- [Firefox Extension](https://docs.google.com/file/d/0B4cvFDDdU8JpTUR3ZVBVeE9jVzA/edit?usp=sharing)
- [Chrome Extension](https://chrome.google.com/webstore/detail/stack-exchange-kbd-button/ignlodfdkifdjomipiamkaaoiokkjlok)

Please report any issues on the [GitHub issue tracker](https://github.com/oliversalzburg/se-kbd-button/issues) and I'll look into it.

  [1]: https://i.sstatic.net/9VpQh.png