Central dispatcher TODELETE

Another variation for MEP22 PR https://github.com/matplotlib/matplotlib/pull/2759 https://github.com/matplotlib/matplotlib/pull/3652
This time using **pydispatch** as central signal event dispatcher, to communicate between `toolbar`, `navigation` and `tools`.
The use of **pydispatch** is just for simplicity, and can be replaced with custom code. But the idea, is to implement a central signal handler, that everybody can `connect` and `send` to.