Replace remaining uses of unsmob_xxx with Xxx::unsmob

Replace remaining uses of unsmob_xxx with Xxx::unsmob

This runs
sed -i 's/unsmob_\(engraver\|global_context\|input\|item\|music\|paper_score\|performance\|performer\|spanner\|stream_event\)\b/\u\1::unsmob/g' $(git grep -l unsmob_ lily)

as the mechanical part of the conversion.  A separate commit provides
the required definitions.