Import positions from Guitar Pro files.

- Corrected several issues with effects like vibrato and tremolo
picking, which apparently can appear on either the beat or an individual
note.
- Corrected issues with importing empty beats (e.g. if there is an empty
second voice).
- It doesn't seem like arpeggio up/down symbols are supported anywhere
in the file format (or, at least GP6 doesn't include them when exporting
to GP5 files)