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`.

I was looking around and I couldn't find anything like that in Figure or anywhere else.

Ithink the best will be to create a mixin class (something like `ViewChangerMixin`) for `Zoom` and `Pan` that deals with the view changes and store the info. Including this (soon to be renamed) `draw` method