Put sort arrows in sprite. Make 100% of th clickable. Align sort indicators. ...

Put sort arrows in sprite. Make 100% of th clickable.  Align sort indicators.  Remove unneeded padding. Props scribu. fixes #15580

git-svn-id: https://core.svn.wordpress.org/trunk@17110 1a063a9b-81f0-0310-95a4-ce76da25c4cd