Use global environment when exec'ing code.  The code now works with either Python 2 or 3.
Merge branch 'master' of github.com:leo-editor/leo-editor
Removed unwanted trace in wvp.exec_code.  Use g.es_exception to report exceptions.
Merge branch 'master' of github.com:leo-editor/leo-editor into vr2-markdown
Phase 2 of vr2-markdown support.  Everything works except slideshow mode.