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 had to revert this in https://github.com/musescore/MuseScore/commit/84aab7041a8a2a9e2e63c61926e3e1e53db62d94 because of http://musescore.org/en/node/41991