- updated to use ttk widgets throughout
- simplified controls, changed go/step/etc. to image buttons
- overall layout is a two-pane horizontal window (controls, stack, status on left, variables on right)
- treeview widget used to replace custom scrolled-listbox stackviewer and custom canvas-based namespaceviewer
- integrated locals/globals into a single treeview on right; can collapse/expand each independently instead of having checkboxes to control whether shown or not

was being used by stack trace in debugger

- button captions now use TkIconFont (a bit bigger than before)
- status line now uses default font, plus derives italic variation for errors

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>