partial fix for #46861: musicxml export lyrics extend

[MarcSabatella]

I originally assumed we lost the MusicXML export for lyrics extenders (underscore) with the fairly recent overhaul from @mgavioli . But actually, it seems this code was removed way back in 2011, here:

http://sourceforge.net/p/mscore/code/4587/

@lvinken , @wschweer : do either of you remember what this might be about? As far as I can tell, in the current code, simply checking l->ticks() works just fine to give us the same export we had in 1.3: a single "extend" element on the lyric itself. No support for the MusicXML 3.0 start/continue/stop tags, but apparently that isn't strictly necessary. I think my PR here should restore 2.0 to the same level of functionality as 1.3, unless there is something I am missing.

Getting the import to work seems it might be more difficult, but at least one user is caring more about export than import and really misses this.

[lasconic]

I'm a bit concerned by the fact that this commit would make the import/export process asymetric since we don't import extend at all. @lvinken any thoughts?

[MarcSabatella]

My concern is that at least one user developing software that relies on our MusicXML export sees this as a significant / showstopper regression from 1.3, which did export the extend tag in this way. And as far as I know, 1.3 did not import it either. See https://musescore.org/en/node/66866

I imagine we could implement a rough version of the import, not supporting the start/continute/end tags and always assuming extend means until the next lyric. I will look into this.

[lvinken]

Marc,Nicolas,

sorry about not responding to your questions, but I have been on vacation and off-line for three weeks.

I have no specific recollection about disabling the extend, although many MusicXML features were broken by the big redesigns after 1.3 and repaired later. This may have been one of those.

In the near future I would like to support for the MusicXML 3.0 start/continue/stop tags and add import and regression tests.