Turtle background picture

[donkirkby]

Add support for the Screen.bgpic() method. One challenge is that the PyCharm canvas only supports PNG graphics, not sure about the browser. The turtle module supports at least GIF and PNG graphics, so either add support for GIF to PyCharm and the browser, or use PIL to convert if it's installed and fail if it's not.

Also check why it currently fails with a weird error:

Live coding plugin error:
Traceback (most recent call last):
  File "/usr/lib/python3.9/runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.9/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/don/.cache/JetBrains/IntelliJIdea2020.1/plugins-sandbox/plugins/livepy/classes/space_tracer/__main__.py", line 6, in <module>
    main()
  File "/home/don/.cache/JetBrains/IntelliJIdea2020.1/plugins-sandbox/plugins/livepy/classes/space_tracer/main.py", line 176, in main
    code_report = tracer.trace_command()
  File "/home/don/.cache/JetBrains/IntelliJIdea2020.1/plugins-sandbox/plugins/livepy/classes/space_tracer/main.py", line 596, in trace_command
    '\n'.join(turtle_report) +
TypeError: sequence item 14: expected str instance, tuple found