Standard getters/setters for axis inversion state

Whether an axis is inverted can now be queried and set using the axes.Axes getters get_xinverted/get_yinverted and setters set_xinverted/set_yinverted.

The previously existing methods (Axes.xaxis_inverted, Axes.invert_xaxis) are now discouraged (but not deprecated) due to their non-standard naming and behavior.