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 agree for the repetitive, I will add a list. 
If the list should be inside axes... this is subject to a different PR not really related to this MEP

Why this draw is here... apart from "it was already there", I have no good answer. It is not actually a `draw` it is more a `update_locators`, and for the moment it is only used by `zoom` and `pan`. 
It doesn't really belong to canvas. What do you think of moving it into `backend_tools.py`, as a helper function `update_locators(canvas)`?