List header search needs a submit button for accessibility reasons

The list header search boxes need a submit button for accessibility reasons.
It shouldn't be shown but should be accessible to keyboard users.

![screen shot 2014-03-13 at 11 21 32](https://f.cloud.github.com/assets/218391/2408750/a6668faa-aaa1-11e3-9938-da860c99ac5b.png)