Fixes leaky savedInputChecked across multiple instances

[daftspunk]

Steps to Reproduce Issue

1. Create two sortables on the same page: List A and List B

2. The options.handle must be specified for some reason

3. Add an event when a checkbox is unchecked inside List A

4. Set event to destroy and replace SortableJS on List B

Problem

All checkboxes on List A are rechecked when List B is rebound (via _nulling)

Expectation

The checkbox should remain unchecked

Proposed Fix

It seems to happen because savedInputChecked is a global instance. So List B's store will contain List A's checkboxes...

The proposed code ensures the checkboxes remain within the scope of the relevant instance.

Related

This may be a solution to #1052's continuation since people are still suggesting it's broken.

It could be why you can't reproduce it, since two SortableJS instances are needed to produce the collision/outcome.