MEP22 Navigation toolbar coexistence

This version of the MEP22 implementation https://github.com/matplotlib/matplotlib/wiki/Mep22#implementation 

The proposed solution is to take the actions out of the `Toolbar` and the shortcuts out of the `Canvas`. This actions and shortcuts will be in the form of `Tools`.

A new class `Navigation` is the bridge between the events from the `Canvas` and `Toolbar` and redirect them to the appropriate `Tool`.

Can you add some documentation to these classes to explain what they do; and/or why/how one should use them?  It would save my brain some pain and for future developers to know what functionality they can expect by using these mixins :).