Feature request: Display 4/2 time as allabreve

[simon_albrecht]

Hello fellow Lilypond fans,

since the Renaissance Alla breve mensura is usually rendered in modern
notation as 4/2 time, I'd appreciate the introduction of a genuine
possibility to display this time signature as slashed c (exactly like 2/2
time). In my opinion, the easiest solution, and well acceptable for me,
were to make this standard behaviour and turn it off by
\numericTimeSignature. However, it would be equally possible to add
another selectable value for the #'style property of TimeSignature.