Fade-in articulation in GP4 and GP5 formats

Adds support for importing the 'fade in' articulation for GP4 and GP5 formats. GP6 support has yet to be created. (Note that the fade out symbol is Guitar Pro 6 specific, I'll bundle that in with the fade in articulation support for GP6 in a future pull request.)
The bravura files shouldn't be changed. If the symbol is defined in SMuFL, it should be in the file already. If it's not, then we have have a problem. So bravura/classes.json is currently unused. You shouldn't modify it.