Rules integration

Scratched my own itch :) This now makes it easy to replace nodereference_url using prepopulate (I wanted users to be able to change the gallery nodereference of an image later, but I only want the custom link to show for authors of the gallery). Much more complex logic could be put in now as well.

Additions to the form

There is a new "Use rules to limit link visibility" check box at the top of the form. To use rules you enable this on a link-by-link basis.

Changes to the database

A new field is added to the db schema called "use_rules". an update_2 is made to update the database. So, Make sure you run update.php!

Rules Integration

This implements one event, one condition, and one action.

1) Event: Custom Links being built - this event fires during _custom_links_build_links(). If $link->use_rule is true and rules is enabled, it will fire off the event, passing the link, global user, and the author user to rules, where any other conditions and actions have the chance to work.

2) Condition: The one condition that was needed besides the ones that come with rules was a check for the link_key, so that you can have this rule effect all links enabled for views or just specific ones.

3) Action: Set the Link Visibility. This takes a boolean value (specifically 0 or 1, not TRUE of FALSE) and assigns a $link->visibility to the link, which is then checked and if the visibility is set and false, then it does a "continue;" and wont show that link. Links are assumed to be visible until the visibility is set to FALSE. The other filters in the link configuration form continue to operate as before.

Here is a screenshot of what the rules UI looks like.

@KuizHack - feel free to reuse this patch, but it's best to get everything approved through the central project so that it will be supported in the future.