Multiple target elements support

At present, each activeselect can have only one target element. This is inadequate in some situations: there are times when one change needs to directly affect multiple other select boxes.

I say directly, because multiple elements can already be affected indirectly through a cascading activeselect setup. However, without multiple target support, such a chain of cascades is limited to being strictly linear, rather than being a branching tree of effects.

The request model for activeselect will not change: only the response model. Here's the current model:

Request data: the currently selected options in the source element
Response data: the new set of available options for the target element

And here's the proposed new model:

Request data: the currently selected options in the source element
Response data: the new set of available options for each target element, grouped by element