fix #291758 - [Musicxml Export] - Tbox and Vbox after measure 1 don't export

[lvinken]

… export

Resolves: https://musescore.org/en/node/291758

Export text frames as MusicXML words and all vertical frames as MusicXML credit-words. Popular request, also required to better support Braille transcriptions.

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[dmitrio95]

Could these new test files be added to tst_mxml_io.cpp to have the relevant tests running on Travis?

[lvinken]

I think that could indeed be done, although I am not quite sure yet how much effort would be required or when I could do that.

[dmitrio95]

Doesn't the existing mxmlMscxExportTestRef already do what is needed for these tests?

[lvinken]

Indeed you are right, I had forgotten about that. In my defence, that function was implemented by Igevorse five years ago :-). Will make the required changes, squash the commits and do a forced push.

[lvinken]

Done, also included some cleanup and a rebase.