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`.

Aye, I noticed. but thanks for the warning :).   As ``git log`` gives me ``bug when navigation was not defined`` from just now, I feel confident I have that part set up.  Now to figure out if I have to do any special build, environment setting stuff, to test with this branch instead of my main install, or if it will just run straight.