Tweaks for the admin bar comment bubble: allow JS updating the count, center...

Tweaks for the admin bar comment bubble: allow JS updating the count, center the bubble when no pending comments, props duck_, fixes #18809

git-svn-id: https://core.svn.wordpress.org/trunk@19156 1a063a9b-81f0-0310-95a4-ce76da25c4cd