MEP22: Navigation by events

This PR is the implementation of MEP22, it supersedes the https://github.com/matplotlib/matplotlib/pull/2759

The main differences are:
* Navigation implement events
* Toolbar connects to Navigation events to change its sate (add/remove tools, change toggle state)
* Toolbar used `Navigation.tool_trigger_event` to trigger tools
* Rubberband and SetCursor are implemented as tools

**Edited**: for clarity
Welcome back, 
The `Toolbar` is not mandatory adding Toolbar to the name is even less clear.
I guess I can add the extra data needed (position)  in `ToolEvent` and remove that class.