Navigation toolbar coexistence event framework

Hmm, I thought it looked pretty clean to me.  I understand your concern that other backends do it differently, looking at your example of ``backend_wx.py``.  If I get your point correctly, you have concern over the backward compatibility, as we only plan to touch just these two backends with this MEP, if so, I completely agree with all of it.  I guess its possible to put in a dirty fix until they have all been modified to follow a standard API, but...

As you say, several big changes will come later, so no point in trying to figure out the right way to do it now.