Allow constructing boxplots over multiple calls.

[anntzer]

Currently, calls to boxplot() set the axis limits, ticks, and ticklabels
assuming that nothing else, and in particular no other boxplot, will be
drawn on the axis; e.g., after

boxplot(np.random.rand(100), positions=[3])
boxplot(np.random.rand(100), positions=[5])

the xlims will be moved to hide the first boxplot, and even after
manually resetting the xlims, the tick at x=3 and its label will be gone
too.

Fix that to make boxplot a bit more cooperative with previous calls:
instead of forcefully setting the axis limits, just update the dataLim
and let autoscaling handle axis limits; also check whether the axis
already has a FixedLocator/FixedFormatter and if so, just append the new
tick locations and labels.

Also rename manage_xticks to manage_ticks (with deprecation), as the
direction it manages depends on the vert kwarg (discussed in #13435).

attn @phobson I guess.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[phobson]

Thanks @anntzer !