Streamlining the imports

This came after reading trhought the code trying to understand the flow of things.

Please correct me if I'm wrong

Figuremanager and FigureCanvas
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If I understood correctly `Figuremanager` and `FigureCanvas` should be reserved for ready to use classes

I removed `Figuremanager` and `FigureCanvas` from 
* `_backend_gtk3.py`
* `backend_gtk3.py`
* `_backend_qt4.py`
* `backend_qt4.py`

Those classes are not usable by as they are they have to be used with `Agg` or `Cairo`,. 

Callbacks setting
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There is no need to duplicate the setting of callbacks, it should remain in `backend_gtk3.py` and `backend_qt4.py`.
The `Agg` or `Cairo` variants don't need to redefine this.