[MusicXML] invisible notations

[rettinghaus]

This adds proper import and export for invisible notations from MusicXML.

NB: I think the original intention of exportInvisibleElements was to allow exporting invisible elements that can not be marked as invisible in MusicXML. However the current solution showed weird inconsitencies when making all elements in a measure invisible. With this approach invisble notations are correctly exported and imported.

[Jojo-Schmitz]

Seems canWrite() can get eliminated from exportmusicxml.cpp entirely.

[rettinghaus]

No, still needed, because it is generally impossible to mark directions as invisible.

[Jojo-Schmitz]

Hmm, where on export is the actual hiding done? Removing the calls to canWrite() took that away as far as I can tell, and I seen nothing that does this now instead

[rettinghaus]

It is done here and was there all the time:

MuseScore/src/importexport/musicxml/internal/musicxml/export/exportmusicxml.cpp

Lines 610 to 626 in 7b242c3

	void Notations::tag(XmlWriter& xml, const EngravingItem* e)
	{
	if (m_notationsPrinted && m_prevElementVisible != e->visible()) {
	etag(xml);
	}
	if (!m_notationsPrinted) {
	if (e->visible()) {
	xml.startElement("notations");
	} else {
	xml.startElement("notations", { { "print-object", "no" } });
	}
	}
	m_notationsPrinted = true;
	m_prevElementVisible = e->visible();
	}

[Jojo-Schmitz]

Ah, I see. Missing in 3.x...