Allow multiple connected sortables on single page

[kartik-v]

Enhance the connected property to be a string or boolean and thus allow multiple groups of connected sortables on the page. With this enhancement, the connected property will follow these rules:

• if set to false or null/empty, this widget will not be connected to any other sortable widget.
• if set to true, this widget will be connected to all other sortable widgets on the page with connected property set to true.
• if set to a string, this widget will be connected with other sortable widgets matching the same connected string value.