Refactor GTK example to use PyGObject instead of PyGTK

[cztomczak]

I've tried installing PyGTK for Python 3 (with pyenv) and I'm having a hard time. Looks like PyGTK is deprecated according to this blog post: End of an Era: PyGTK. The alternative is to use PyGObject.

Here are some examples:

• gtk-demo.py
• cairo-demo.py
• signal.py

Related issues:

• Create examples/ root dir - simplify and unify examples to work on all platforms #207 - examples/ root dir
• Use external message pump when calling MessageLoopWork (to fix bugs and improve performance) #246 - gtk example should use MessageLoop().

See also the Refactor Examples project.

[cztomczak]

GTK example is so much simpler now, see examples/gtk.py. Tested with python 2 and 3. Closing.