MusicXML: Implement the symbol attribute of time signatures

MusicXML: Implement the symbol attribute of time signatures

-) Fix time keeping (i.e. use the cached value for the current time sig
   and thus the current measure length!): revert to
   self.get_named_attribute ('time')
-) Implement single-digit time signatures
-) Implement C vs. numeric time signatures; Change the signature style
   to '() only if a C or C/ would be printed (i.e. 2/2 or 4/4).
   By default, MusicXML uses numeric signatures (style '()), while
   LilyPond's default is 'C, so we have to add some manual overrides
   in those cases of 2/2 or 4/4 where no symbol is given in the
   MusicXML file