Resizable: custom handles not supported for multiple elements

Assuming issue #4310 is resolved (or you manually add ui-resizable-handle and ui-resizable-s classes to your custom south handle), the custom handle will still not work properly if you selected multiple elements to be resizable.

For example:

$(".box").resizable({handles: {"s":".footer"}});

with html snippet:

<div class="box">

<div class="footer ui-resizable-handle ui-resizable-s"></div>

</div>

<div class="box">

<div class="footer ui-resizable-handle ui-resizable-s"></div>

</div>

Only the first box will have a usable south handle. It seems like the footer element for the first box is being configured as the handle for all all boxes.

Attaching a potential fix.