Improvements to hover and focus states on the adminbar:

* Add a clear hover state for top-level links in the adminbar.
* Add :focus styles to the adminbar for keyboard accessibility.
* Ensure that the username link in the profile submenu gets a :hover style.
* Add adminbar hover styles to color schemes.
* Set -webkit-font-smoothing to subpixel-antialiased on the adminbar so that Safari doesn't switch to standard antialiasing on mouseover.

Fixes #26152, props ryelle.


Built from https://develop.svn.wordpress.org/trunk@26407


git-svn-id: http://core.svn.wordpress.org/trunk@26307 1a063a9b-81f0-0310-95a4-ce76da25c4cd