WP-admin.css cleanup + border-radius improvements [patch]

[Simek]

Some improvements and cleanups for WP-admin.css.

Main change:
I use shortest formula for "border-radius" - it works very well and use less characters.

Based on latest revision - patch included.

[azaozz]

(In [8723]) wp-admin.css cleanup, props Simek fixes #7584

[azaozz]

(In [8725]) Only Mozilla seems to support shorthand for border-radius. Reverting the rest, see #7584

[Simek]

I test it in Safari 3.1.2 under Windows and most of radius working good but not all. It need deeper analysis.

Also pure CSS3 property support shorthand for border-radius.

[azaozz]

The "pure" CSS3 border-radius doesn't seem to include a shorthand: ​http://www.w3.org/TR/css3-border/#the-border-radius (not that it's supported by any browser anyways).

It has 2 values for each corner, so a shorthand would have 8.