MEP22 first draft (DO NOT MERGE)

[fariza]

Closed in favor of #2759

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 #2759

[tacaswell]

I have made a MEP22 label to stick on issues that I think are related to this.