fix #307271 : [Musicxml Export] - Wrong tuplet calculation in two-note tremolo

[AntonioBL]

Resolves: https://musescore.org/en/node/307271

This fixes a bug in tuplet import (tick was not set, so when deleting a tuplet in a file imported from xml there was corruption) and in xml import of a two-note tremolo at the beginning of a tuplet. It adds the optional tuplet-actual and tuples-normal fields in MusicXML export for compatibility with some Braille exporting softwares.

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[AntonioBL]

The case of nested tuplet import is not dealt with, since import of MusicXML nested tuplets is not implemented yet. (But export works ok)