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.)
Not sure if this is necessary. The default behavior of articulations could be enough.