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`.

Stray comma in class inheritance definition.

Also, I haven't used GTK before, so just curious about the reasoning to switch from ``Gtk.Toolbar`` in ``NavigationToolbar2GTK3`` to ``Gtk.Box`` here.  It took me a bit by surprise ;).