static/js/dualListbox: add multiple selection

Add multiple selection using shift + click to dualListbox. Also prevent accidental moving elements by double clicking them while holding shift.

This multiple selection method creates an anchor on the clicked element. While holding shift and clicking on another element, it selects all elements in the inclusive range, making this clicked element a focus element. That allows you to shrink the range, when clicking (while holding shift) on the element before the focus element.