Some Admin AJAX tweaks

[mdawaffe]

Both a defect and an enhancement.

Attached:

1. Remove wp_clean_ajax_input() from admin-ajax.php. I can't remember why it's there. It may have been necessary at one point, but now it just urldecodes strings that are already udecoded. To test, try adding the category "no amp here: %26" via ajax cat. Pre-patch, you'll see: "no amp here: &". Post-patch you'll see "no amp here: %26".
2. Allow plugins to use admin-ajax.php by firing 'wp_ajax_$action' if there's no hardcoded case.
3. Use full path for AJAX response handler to avoid nastiness.
4. Allow plugins to use listMan library with their own AJAX response handler.
5. Make listMan object a little more usable to plugin JS.
6. Step toward getting rid of nasty eval()s.

tweaks to list-manipulation-js.php and admin-ajax.php

Oh - forgot: the changeset number for list-manipulation-js.php in script-loader.php should be updated when this goes in.

(In [4042]) Admin AJAX tweaks from mdawaffe. fixes #2969