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 personally found this method name a bit confusing when I first read it.  I still find it a bit cryptic but I understand it now.  I think the confusion for me came with the word ``add``, perhaps something like ``setup``, or ``init`` would make this clearer?  Perhaps in combination with stating it as ``message_area``, as this function does more then just a message.

What do you think of ``_setup_status_area``?