Add a hook in sortables to generate the list of sortables

Hey,

I have created a nested sortable list which can only be 1 level deep. The cleanest way I've found to enforce this is to add hook into the refreshItems() which generates a different list of sortable items depending on what item I'm picking up.

Example, for my use:

if item contains a list

return list of sortables on root of list

else

return all sortables in list

Attached is a patch I've made from my changes on the most current svn trunk.

-Ryan