A command started in one drawing kept running after a programmatic NEW or OPEN made another
drawing current. Finishing it there recorded an undo step holding the first drawing, so undo in
the new drawing brought the other drawing back. Session::new_drawing and open_drawing now end a
running command before the new drawing becomes current, as CLOSE and document.switch already do
and as starting NEW from the command line did: its undo step lands in the drawing it started in.
With no command running, nothing changes (the previous drawing keeps its selection).
