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.)
I would create two different articulationType, fade in and fadeout. u/dmarcato are for below and above the staff.