More flexible capabilities model

[scribu]

Currently, to create a connection between two items, you need to be able to edit both those items.

This doesn't always makes sense. For example, a site owner may want to allow authors to create a list of related articles, some of which might not be written by them.

Relevant support threads:

http://wordpress.org/support/topic/posts-to-page-connections-for-authors

http://wordpress.org/support/topic/private-post-connections-only-show-to-admin

http://wordpress.org/support/topic/admin-box-not-being-displayed-in-cpt-user-connection

[imjjss]

another example--users mostly do not have the capability to edit their favorite posts.

[scribu]

If someone wants to take a stab at this, I'll be happy to review the pull request and merge.