Welcome Screen: update column widths to show 3-col view on 1280 wide screens....

Welcome Screen: update column widths to show 3-col view on 1280 wide screens. Props helenyhou chexee, see #11651

git-svn-id: https://core.svn.wordpress.org/trunk@19341 1a063a9b-81f0-0310-95a4-ce76da25c4cd