Use CSS3 linear gradients instead of background images

[scribu]

There are several places in the admin where CSS3 gradients could be used instead of images.

This will speed up page load and generally be easier to work with.

Example cross-browser implementation: ​http://webdesignerwall.com/tutorials/cross-browser-css-gradient

Note: The files added to wp-admin/includes/update-core.php should be removed from svn (sadly, svn diffs don't capture this).

[scribu]

More ideas: ​http://www.smashingmagazine.com/2011/04/21/css3-vs-css-a-speed-benchmark

[scribu]

buttons

[ocean90]

Duplicate of #16461.

[scribu]

Rats...