Stop event propagation on sortable item's dragstart

Ignore commit 6d435fc

Stop propogation of dragstart event if caused by a valid handle so as
to not cause possible parent listener to prevent the default drag action.

Example use case:
This enables the ability to rearrange the order of Items and the Subitems within them, although the Subitems within an Item cannot be dragged to other Items.
Item 1
SubItem 1.1
SubItem 1.2
SubItem 1.3
Item 2
SubItem 2.1
SubItem 2.0
Item 3
SubItem 3.1
SubItem 3.2