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`.

@fariza Yes, I see what you mean.  Very tempting indeed, for me because of the desire to keep backward comparability, knowing that when other problems get solved, it may well in turn enable a better/clearer API for tools.

I have made quite a bit of progress, so I shall keep going a bit further to see the end result, if I may.  Not too much further to go :).