Single axes artist

This adds check to ensure that you don't add an artist to more than one axes (which does not work because the transform stacks clobber each other).
It already re-defines the property on https://github.com/matplotlib/matplotlib/pull/3835/files#diff-c3ce78ddcc28c54e04ade080c149c1b1R397 where it is overloaded to mean 'the list of axes this figure contains'.