Drag & Drop of Presenters to Documents/Pasteboard

[gbracha]

We should resurrect the Pasteboard feature we had in Squeak. It may or may not be necessary to define an actual Pasteboard - documents may serve that purpose. To recap, presenters could be dragged to a pasteboard and dropped there; in reality, it wa sthe subject that was dropped, and a presenter for it created. This allowed one to place different views in proximity. One can achieve this with documents by opening amplets, but that requires knowing the right code for the amplet(s).

To fully come in to its own, one would want tiling (see issue #131 ).

A variation on this would be dragging amplets within a document, where one would expect them to be moved rather than copied.
Full copy/paste for amplets would be nice too.