Guitar pro import

A couple of fixes to importing the GP3 format:

- It's possible to create specify notes in TuxGuitar with a really high pitch which can cause a crash in Musescore. Guitar pro handles this error by setting the fret number to 0 and displaying the note anyway. We do the same as it seems to me the sensible way to go.

- Improved the way that dotted notes are handled, could cause some problems when placed at the end of a bar in the GP3 format when attached to a glissando.