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
Just going to say I will make the PR at the weekend when I look at it properly, but I see you have beaten me to it.  If I find anything else I will go straight to PR.