Admin: refactor `bp_core_admin_tabs` and create a `get` function.

This is a development aid for contributors experimenting with modifying different parts of BuddyPress screens in side wp-admin.
The nav tabs were defined and templated in the same function, making it hard to modify the output.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9205 cdf35c40-ae34-48e0-9cc9-0c9da1808c22