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
How about a compromise?  Renaming this attribute as ``default_keymap`` gives us more flexibility, allowing us to override this default during tool creation.  This also means that user can prevent tools from overwriting keymaps of belonging to other tools if a collision occurs.