Toggle drag canvas visibilty during drag.

Making the background transparent gets our hit tests so you can drag off
a node and have the OnMove stop being called. But you then have to set
the canvas brush to null to get the clicks to go down to the bench.
Comments to explain canvas visibility.
Little cleanup of the bench xaml.
Create a command for nodeFromSelection and call it.
Test if can execute
Now calling NodeFromSelectionCmd from context menu.
Return true for can execute on node from selection.

This is until we can figure out a clever way to get the selection update
events after the initial call to CanExecute.