Improvements to backside emerging

And drag and drop
Top-level backside operational

Other improvements to backsides and frontsides for generality and
clarity.
First attempt to use datatransfer

JSON like code is beginning to be integrated for drag and drop between
windows.
Data transfer working!

At least numbers between Chrome windows.
Box JSON and cross window drag and drop

Boxes now produce and parse JSON objects. Drag and drop across windows
working. Positioning the newly dropped widgets remains to do.