layout improvements....

As discussed on twitter with @mwaskom and @anntzer - it would be niece to be able to take x or y out of the layout independently (i.e. in Artist.set_in_layout(x=False, y=True) For many artists this is probably not too hard, but will take a little finesse. Probably not too hard to do in a backwards compatible way.

Also, ax.get_tightbbox should let ax.xaxis and ax.yaxis be taken out of the layout.