Re: Issue 3491: Add \displayMarkup command. (issue 12732043)

[dak]

This looks fine for committing to me.  There are several obvious
followup issues/commits making a lot of sense afterwards, however.

One is the obvious complement with \displayLilyScheme which makes sense
to do before the others, namely providing a notice in
Documentation/changes.itely and making some regtest(s).  I'm not sure I
have a good idea for a regtest, though.  Something like
input/regression/display-lily-tests.ly seems like quite a bit of
overkill.

https://codereview.appspot.com/12732043/