Improved SimpleGui, added Tkinter support

loading the backends was handled in a really silly way. for instance, in the
previous version _all_ these backend would get loaded. serious overhead... so,
pythonocc is now more selective about loading the backend and does so in a
logical order.

fixed the menu bar issue, and added a method that centers the window and sizes the
window, depending on the resolution of the screen.

Also adds an optional argument to init_display to overload the default backend.
Updated qt_demo.py and wx_demo.py examples to the changes to the backend management
Merge pull request #133 from tpaviot/jf/gui-improvements

Improved SimpleGui, added Tkinter support