Using the new buttons:
* Button classes are now stackable.
* All buttons should use a base class of "button".
* Buttons default to the gray style (formerly "button-secondary"). Buttons can add a style by adding additional classes. To make a primary button, add the "button-primary" class.
* Buttons can be rendered in various sizes. In addition to the default size, you can add "button-large", "button-small", or "button-tiny".
For backwards compatibility reasons, "button-primary" and "button-secondary" both work as standalone classes.
get_submit_button() has been adjusted to handle shorthand button classes (i.e. button classes can be passed without the "button-" prefix).
props lessbloat, helenyhou, trepmal, nacin. see #21598.
git-svn-id: https://develop.svn.wordpress.org/trunk@21789 602fd350-edb4-49c9-b593-d223f7449a82
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