Adds 'widget' class to all BP widget wrappers

Manually adding the 'widget' class ensures that BP's styles and JS are applied
to widgets in themes that do not natively use the 'widget' class for wrapping
widgets (such as Twenty Ten).

Fixes #4869

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7210 cdf35c40-ae34-48e0-9cc9-0c9da1808c22