Define BP_Group_Extension screen callbacks non-statically

These callbacks should not be defined statically, because then the $this
keyword is not available in the callbacks. This causes backward compatibility
issues, especially with PHP 5.2.x, where it results in fatal errors.

Fixes #5126

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7332 cdf35c40-ae34-48e0-9cc9-0c9da1808c22