Add a view filter to omit the drums when importing Guitar Pro files.

[cameronwhite]

Related to #290, the imported drum track is currently not very useful, so it would be good to be able to easily hide it. Or perhaps we should just skip importing it until drum scores are implemented?

[psyomn]

Would it make sense to import and hide, so that the user in the future, would not have to re-import files? I don't know what kind of file format nightmare this might contribute to maintainability though.

[cameronwhite]

I think I'd lean towards skipping - right now a Score has no notion of drums, so the result is just whatever guitar properties the importer happens to find (and it wouldn't suddenly start working for old imported files if drums support was added). For .gpx files I think all the notes show up as the 0 fret, so a lot of data is definitely being missed.