Cleanup for drawstyles.

[anntzer]

1. Remove unused entries from STEP_LOOKUP_MAP (plt.step already
   normalizes drawstyles to step-* format).
2. Clarify the error for invalid drawstyles.
3. Fix handling of step-* drawstyles by the Qt options editor.

[tacaswell]

https://travis-ci.org/matplotlib/matplotlib/jobs/136321898#L1084

Apparently that feature got in with out enough tests.

[anntzer]

Fixed. I chose to normalize the input of fill_between to steps-*.

[anntzer]

Actually, can we backport this to 2.0? Up to 1.5.1, opening the figure options editor after making a plot of drawstyle "steps-post" (for example) would print an error and mangle the drawstyle, which is bad, but still much better than the situation in 2.0b3 where an exception in thrown in the event loop (... and by default, PyQt5 aborts the program in such a case).

[tacaswell]

Does the whole PR need to go back, or just the changes to figureoptions?

[anntzer]

I think the latter should be enough.

[anntzer]

@tacaswell should I open a new PR with just the changes to figureoptions? Will that lead to merge conflicts when merging 2.0 back into master (if we ever do that?).
TBH I think the PR is small enough that splitting it is a bit overkill but either way works for me.
(Again, note that the change to figureoptions removes a possibility for a fatal exception in the Qt5 backend.)

[tacaswell]

Can you do a PR with just the figureoptions change?