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 would have to look into the various usecases for matplotlib, to look at entry points, but your example of the gtk3 backend could work nicely if we wanted backend customised tools/toolsets added... for example your ``CopyToolGTK3 `` in the navigation example.