Slides functionality, split up Guitar Pro implementation, grace/simile marking support in GP6

Slides implementation was forked from my current GP6 branch to access some other exishting functionality, so this PR includes other changes as well. I'll make a PR for my current GP6 branch when this is merged so that everything can get tested by others.

Edits outside of the guitar pro implementation are those to chordline.cpp - there is now a _straight property which when set to true will create straight lines instead of bezier curves when drawn (glissando implementation is now no longer modified). CMakeLists has also been updated to include the additional guitar pro implementation files.