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`.

We have duplicated code here, not so nice for maintenance, should replace with something like:
``` python
self.set_tool_keymap(name, validate_stringlist(tool_cls.keymap))
```