Improve MusicXML exporter (4)

[cecilios]

This PR fixes all issues detected in MusicXML import/export round-trip automated tests, using MusicXML test sets from recordare and Lilypond.

Fixes in MusicXML exporter

• Element <measure-numbering> was not exported. Fixed.
• Elements <word-font>, <lyric-font> and <lyric-language> were not exported. Fixed.
• Bad ordering of <score-instrument> and <midi-instrument>. Fixed.
• Attribute implicit in <measure> elements not exported. Fixed.
• In some cases, dynamics are exported in different order and get associated to a different note. Fixed.
• The exporter generates empty <staff-details> elements. Fixed.
• Wedge attributes were not exported. Fixed.
• Orientation not exported in <tied> elements. Fixed.
• Spanner notations not exported in rests. Fixed.
• Error in slurs when stop defined before start. Fixed.
• Barline 'double repetition heavy-heavy' was not correctly exported. Fixed.
• Fixed bug in generation of <page-layout> element.
• Mandatory <chromatic> child not exported in <transpose>element. Fixed.

Other changes

• Fixed a bug in the algorithm to build the measures table (MeasuresTableBuilder object).
• MusicXML importer didn't correctly import <staff-details> elements. Fixed.
• MusicXML importer now imports <sound> elements of type damper-pedal, soft-pedal and sostenuto-pedal.

The MusicXML exporter can be now considered fully functional and all imported content is re-exported without loses. Please, report any issue you detect. Thank you.