Allow _wp_get_comment_list to handle custom comment types

[aaroncampbell]

Plugins (like ​tweetbacks by Joost) can add custom comment types. While these can be easily used in most places, but not in _wp_get_comment_list(). Actually, on the edit-comments page you can use the admin_comment_types_dropdown filter to add additional comment types to the dropdown, but you can't actually filter by them.

I'm attaching a patch to fix it based on the existing usage where types is a string. However, I'm wondering if we should allow type to be either a string OR an array, which would allow you to specify multiple types.

[aaroncampbell]

To test you'll need to have some comments in the comments table that have a comment_type other than the defaults (empty, 'pingback', and 'trackback'). If you have the ​tweetbacks plugin installed, you probably already have some with the comment_type 'tweetback'.

Once you have some comments in there to test with, go to /wp-admin/edit-comments.php?comment_type=____, filling in with your new comment_type.

[scribu]

Tested on the 2.8 branch with regular comments, pings, trackbacks and two custom types: 'foo' and 'bar' and it works as expected.

[azaozz]

(In [11748]) Allow _wp_get_comment_list to handle custom comment types, props aaroncampbell, fixes #10463

[azaozz]

This slightly interferes with the Trash functionality.

[ryan]

This breaks comment search.

[ryan]

(In [11954]) Special case comment type 'all'. Fixes comment search. fixes #10463