Expose javascript replacements to other modules

Currently it is very difficult for other modules to detect when modules (such as jquery_update) alter javascript in the theme layer. We ran into this problem with ctools in this issue #745468: ctools_ajax_command_scripts() in ctools_ajax_render() causes unwanted scripts to be loaded (and potential js conflicts) where code was actually committed that specifically works around jquery_update. We came up with a hook that we hope other modules in contrib can use which exposes these replacements.

I have implemented it for jquery_update and attached the patch. Hopefully when we get this committed we can remove the jquery_update workaround from ctools.