time signature positioning for non-standard staves

time signature positioning for non-standard staves

the only non-standardness the old code catered for is altered line-count.
the aim here is to position time signature similarly with any of the following:
\override StaffSymbol #'line-count = #'4
\override StaffSymbol #'line-positions = #'(-2 0 2 4)
\override StaffSymbol #'line-positions = #'(-4 -2 0 2)