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`.

This doesn't feel like the right place for this.

I propose:
1. Moving this into a separate method ``NavigationBase.addTools``.
2. Adding the default tools later, as it feels like a separate concept.  The vessel and what we choose to fill it with.  (This also hinders my other work).

I have applied point 1, looking for the best place to do 2.  I shall submit a PR with just this later today.