Improve the admin color scheme picker:
- Easier to extend.
- Remove the "drop-down" look, show all choices inline.
- Some PHP and JS cleanup.
Props ryelle, fixes #26336.
Built from https://develop.svn.wordpress.org/trunk@26506


git-svn-id: http://core.svn.wordpress.org/trunk@26400 1a063a9b-81f0-0310-95a4-ce76da25c4cd