fix #331007: [MusicXML import] support tie element

[lvinken]

Resolves: https://musescore.org/en/node/331007

Create a tie on MusicXML import when either a tie or a tied element is present.

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[Jojo-Schmitz]

@lvinken: My backport to 3.x fails an mtest, "testUnterminatedTies" (see here), seems related to this change. any idea how to deal with it?

That test doesn't seem to exist in master, see:

MuseScore/src/importexport/musicxml/tests/tst_mxml_io.cpp

Lines 255 to 256 in a623af3

	void unnecessaryBarlines() { mxmlImportTestRef("testUnnecessaryBarlines"); }
	void unusualDurations() { mxmlIoTestRef("testUnusualDurations"); }

I see, that test isn't in 3.x either, but had been added with #8556 into the 3.6.2_backend_musicxml branch.

[lvinken]

It is quite unfortunate that MusicXML import in the backend and the application itself are not in sync ...

[Jojo-Schmitz]

Yes, it is. And unfortunate that the author of those is no longer around.