Compromise between a mix of razor-sharp corners and gratuitously over-rounded...

Compromise between a mix of razor-sharp corners and gratuitously over-rounded "safety corners" by standardizing around 3px border-radius (mouse-nibbled corners). see #17324

git-svn-id: https://core.svn.wordpress.org/trunk@17820 1a063a9b-81f0-0310-95a4-ce76da25c4cd