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.
I'm not sure, but this could be based on my comment in the issue thread that by default, I expect straight chordlines added via palette tobe laid out exactly the same as curved, only differences beting straight versus curved.  So if any GuitarPro-specific munging is to be done, it should be done during the import somehow.  But I'm still very unclear on what exactly the special requirements for GP are.