first steps toward getting the sample output to run:
 - create a GTK window (works)
 - create a dummy TA window, which automatically sets up the canvas, but
   without menus and palettes (works halfway)
no more errors when the output code opens a window
give the main window a real class (DummyTurtleMain)
 - the constructor sets up the window and the widgets
 - there is a method for exiting properly
 - the DummyTurtleMain object holds the DummyTAWindow instance
- get rid of toolbar
- add a main() function and some functions representing sample block
  stacks
- clean up the code and add documentation