FigureFrameWx constructor, subclasses, and get_canvas

The FigureCanvasWx constructor gained a canvas_class keyword-only parameter which specifies the canvas class that should be used. This parameter will become required in the future. The get_canvas method, which was previously used to customize canvas creation, is deprecated. The FigureFrameWxAgg and FigureFrameWxCairo subclasses, which overrode get_canvas, are deprecated.