MEP22 update

Hello everybody.

I just did a little update of the PR https://github.com/matplotlib/matplotlib/pull/2759
This is the implementation of the MEP22.

I would like to have some feedback on this.
It is working and rebased so should merge cleanly.

There is an example in examples/user_interfaces/navigation.py that shows the basic functionality of tools and toolbar reconfiguration and management.

Any comment is welcomed.

Thanks
Federico