Changes to Qt backend class MRO

To support both Agg and cairo rendering for Qt backends all of the

non-Agg specific code previously in

:class:`.backend_qt5agg.FigureCanvasQTAggBase` has been moved to

:class:`.backend_qt5.FigureCanvasQT` so it can be shared with the cairo

implementation. The :meth:`.FigureCanvasQTAggBase.paintEvent`,

:meth:`.FigureCanvasQTAggBase.blit`, and

:meth:`.FigureCanvasQTAggBase.print_figure` methods have moved to

:meth:`.FigureCanvasQTAgg.paintEvent`, :meth:`.FigureCanvasQTAgg.blit`, and

:meth:`.FigureCanvasQTAgg.print_figure`. The first two methods assume that

the instance is also a :class:`QWidget` so to use

:class:`FigureCanvasQTAggBase` it was required to multiple inherit

from a :class:`QWidget` sub-class.

Having moved all of it's methods either up or down the class hierarchy

:class:`FigureCanvasQTAggBase` has been deprecated. To do this with

out warning and to preserve as much API as possible,

:class:`.backend_qt5.FigureCanvasQTAggBase` now inherits from

:class:`.backend_qt5.FigureCanvasQTAgg`.

The MRO for :class:`FigureCanvasQTAgg` and

:class:`FigureCanvasQTAggBase` used to be ::

and ::

respectively. They are now ::

and ::