* Simplifies button selectors. Maintains backwards compatibility for several classes and makes creating new button styles considerably easier.
* Adds buttons documentation.
* Raw input submit/reset/button types now use default browser styles (instead of half-styled borders that did not look like buttons).
* Moves basic button color standardization to the beginning of wp-admin.
see #21598, #22197.
git-svn-id: https://develop.svn.wordpress.org/trunk@22239 602fd350-edb4-49c9-b593-d223f7449a82
Adds `wp-includes/css/buttons.css` to consolidate buttons files and make them available in a modular fashion. Buttons are included automatically with the `colors` styles, and can be included independently by running:
`wp_enqueue_style('buttons');`
fixes#22197, see #21598.
git-svn-id: https://develop.svn.wordpress.org/trunk@22236 602fd350-edb4-49c9-b593-d223f7449a82
