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.)
If you know nothing about the ove format, I wouldn't change the code in importove.cpp, ove.cpp and ove.h