Improve Group's Edit WP Admin Bar menu

Use the group's manage options nav to populate the different edit links of the group's Edit WP Admin Bar.
Introduce a new parameter {{{show_in_admin_bar}}} for the edit screen property of the Group Extension API to allow plugins add their edit link into this Admin Bar (if this parameter is set to true).

props boonebgorges

Fixes #6002

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9131 cdf35c40-ae34-48e0-9cc9-0c9da1808c22