Assistant Control Widget duplication offset and z-position reordering.

I have implemented control widget duplication offsetting as suggested by @emmetoneill in !2292 (closed). In addition to the original click-drag duplication ability, the user can now simply click-release the duplication button and the new control widget is offset slightly from the original, avoiding cumbersome overlap.

Adding this improvement required a minor refactor of the event processing code, which could not distinguish which control widget was being clicked on when widgets overlapped. The redesign now utilizes the element order in the assistants list to represent z-ordering. The interaction code uses the z position of the control widgets to determine which widget the user is actually interacting with and to select and bring the widget in front of all other widgets.

I have also made the duplication offset independent from the canvas resolution and zoom, so the offset is constant no matter what canvas resolution or zoom level the user is working with.