Accept bare pitches/durations as music arguments to markup commands

This adds a tiny bit of the smart argument handling music functions have to markup commands. Namely, if a command expects a Scheme argument (i.e., the predicate is not markup? or markup-list?), and { <pitch> } or { <duration> } is passed, the music interpretation as note event of those constructs is tried as well against the predicate. The most obvious use case is

\markup \rhythm { 8. }