Restore legacy widget CSS classes for backwards-compatibility.

r6865 changed all widget CSS classes to 'buddypress'.

However, we have to be considerate to themes that are already using the
older widget CSS classes from BP 1.6 and lower.

This commit brings back these CSS classes.  See #4869.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@6938 cdf35c40-ae34-48e0-9cc9-0c9da1808c22