Adding basic Shift support + some small changes to difficulty detection

Some code was modified to add Shift support (up to 4K+). So far, the modifications are a little bit convoluted, requiring that separate Shift modes be relegated to different stepstypes (2K+ and 4K+ are relegated to dance-routine and dance-couple, respectively), but the code itself was tested without bugs, using some of my note charts as test subjects.

Other minute changes include shifting LT's mode from Beginner to Easy, mostly for personal convenience (having all of the difficulties together with easy, medium, hard, challenge rather than skipping easy).