MEP22 first draft (DO NOT MERGE)

This is the first draft for the MEP22, 

The idea is just to lookat the new organization of stuff. The documentation will come later I promise.
**The only backend working is GTK3, everything else is broken**

Just one example for the moment `examples/user_interfaces/navigation.py`

For implementation structure https://github.com/matplotlib/matplotlib/wiki/Mep22#implementation

For the version that keeps the NavigationToolbar classes https://github.com/matplotlib/matplotlib/pull/2759/