Draggable: Clarify comments and whitespace in connectToSortable
Draggable: Refresh sortables when draggable is added or removed

Since a sortable grows or shrinks when a draggable element is added
to it, refresh the cached positions of sortables whenever an element
is added or removed from the sortable.

Refs #9675
Draggable: Whitespace and naming cleanup of connectToSortable
Sortable: cancelHelperRemoval only considers helper, not placeholder

Refs #9675
Draggable: Add battery of tests to cover connectToSortable

Refs #9481
Refs #9675