make metronomeMarkFormatter more flexible (issue 327620043 by address@hidden)

[dak]

I think this approach is a rather bad tradeoff between additional
context properties and actual flexibility: it still only allows for a
very limited subset of metronome marks.

I think it would make sense to check all forms a \tempo mark currently
can take and then see what principal forms those can assume.  Then, one
needs to look how many basically different format functions it makes
sense to specify for those forms and pass those the _raw_ information in
the \tempo mark, making them responsible for proper formatting.

That's for formatting: probably something needs to be there in parallel
for calculating the moments-per-minute value.

Maybe we want something like

\tempo { \tuplet 2/3 { 4 8 } } = 45

to work as well?  That would require yet another hook.

https://codereview.appspot.com/327620043/