tight/constrained_layout no longer worry about titles that are too wide

*tight_layout* and *constrained_layout* shrink axes to accommodate

"decorations" on the axes. However, if an xlabel or title is too long in the

x direction, making the axes smaller in the x-direction doesn't help. The

behavior of both has been changed to ignore the width of the title and

xlabel and the height of the ylabel in the layout logic.

This also means there is a new keyword argument for `.axes.Axes.get_tightbbox`:

``for_layout_only``, which defaults to *False*, but if *True* returns a

bounding box using the rules above. `.axis.get_tightbbox` gets an

``ignore_label`` keyword argument, which is *None* by default, but which can

also be 'x' or 'y'. These API changes are public, but are meant to be

mostly used internally.