Put widget titles through WP's widget_title filter before assembling markup

This will provide a more predictable way to modify widget titles in the same
way as WP's core widgets.

Fixes #4299

Props magnus78, merty

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6999 cdf35c40-ae34-48e0-9cc9-0c9da1808c22