Skip over unnecessary data in dotted notes and better slide (chordline) layout for GP files

Fixed potential issues stemming from dotted note information in old versions of guitar pro. Also a new slide layout for guitar pro files (see issue [#392391](http://musescore.org/en/node/39231)), fixes spacing problems described in the issue.
this look weird. The path creation should be done in layout() instead. That would make this unecessary, also the order of setting the line type and straight would be irrelevant. What do I miss?