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.
Oh that's interesting, thanks for letting me know. Could be a "tux-guitarism" in play here. I'll check this out.