The clear text button in textboxes doesn't match the textbox foreground

This looks funny if you change the foreground of the textbox/passwordbox. See attached screen shots.

![textbox_before](https://f.cloud.github.com/assets/631724/64605/b7407dfa-5e52-11e2-80b8-1db64742a905.png)

![textbox_after](https://f.cloud.github.com/assets/631724/64606/ba7a38bc-5e52-11e2-8a76-57fceb7a2be3.png)

This PR fixes that issue.