Fix #18158: Do not export invisible text lines

[rettinghaus]

closes #18158

[oktophonie]

Shouldn't the line be exported as invisible, rather than not exported at all?
Also, with this PR if I create an export a file containing (for example) a PM marking with 'Show line' turned off, when re-importing the resulting MusicXML file, MuseScore complains that it is invalid.

[Jojo-Schmitz]

How about

    _xml.startElement("direction-type");
    if (!tl->lineVisible())
        _xml.tag("dashes", { { "dash-length", 0} });
    } else if (isDashes) {
        _xml.tag("dashes", { { "type", type }, { "number", n + 1 } });
    } else {
        _xml.tagRaw(QString("bracket type=\"%1\" number=\"%2\" line-end=\"%3\"%4").arg(type, QString::number(n + 1), lineEnd, rest));

    }
    _xml.endElement();

Something similar is used for invisible (de-)crecendo lines

[Jojo-Schmitz]

Probably requires the import to handle this too

            const qreal dashLenght { _e.attributes().value("dash-length").isEmpty() ? -1.0 : _e.attributes().value("dash-length").toDouble() };
            if (ashLenght != 0) {
                b->setLineVisible(false);  // hack: export encodes invisible dash lines with a dashLength of 0
                b->setLineStyle(Qt::DashLine);
            }

[rettinghaus]

@oktophonie There isn't such a thing as invisible lines in MusicXML. The bracket and dashes are missing the print-object attribute.
I'll recheck the validity issue.

[rettinghaus]

@oktophonie Thanks for testing. The previously "fixed" export of invisible (de)crescendo lines showed the same problem. Now everything should be as expected.