Added svIndex attribute

[no1lov3sme]

Attribute "sv-index" can be used for paginated list of draggable items (should be set to sv-root element). Start index should be provided.

Example: sv-index="{{currentPage > 1 ? (currentPage - 1) * elementsPerPage : 0}}".

[cklemming]

Great improvement!

Tip: To sort between paginated pages, you can use the following strategy:

1. On sort, record the startIndex (based on the current page)
2. Display all items in the list (remove pagination temporarily during sort)
3. Use startIndex to grab the dragged item's index, but do not use it for the targetIndex