[MusicXML] Import layout information such as Measure width

MuseScore bcc0170af4

Currently MuseScore write measure width but doesn't read it when importing MusicXML. As a result, the layout algorithm of MuseScore is applied and it might happen that a measure ends up in a new system alone. It would be a great improvement if MuseScore could consider this width. There is probably quite some work to do in the core to be able to specify the width of a measure... and to avoid the colisions.