Import part-symbol from MusicXML

[rettinghaus]

Currently all multi-staff parts receive a brace as grouping symbol from MusicXML ignoring the part-symbol element (available since verison 2.0) that defines a specific grouping symbol for the part and states which staves should be connected by bar lines.

The element should also be read and a child staffGrp should be added with the respective grpSym.