Fix combo-box dropdown button height for WP 3.9.x. Looks good in Chrome, Opera and the latest IE. The button is too tall in IE 8 because the height of most input fields is not fixed. 

TODO: Consider explicitly setting text field height.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@946886 b8457f37-d9ea-0310-8a92-e5e31aec5664