Show Builder - allow dragging items between Scratchpad & Saved Items

depends on #344

Version 2 functionality

Allow dragging items between the Scratchpad and the Saved Items boxes.

The reordering rules for traffic events should apply here as are described in #344, that is:
* you can put a traffic event anywhere in the Saved Items list, even if it's out of order with other traffic events
* But, traffic events in the Scratchpad have to appear in the proper order of the underlying scheduled event