Allow independent adjustment of minimum length for spanner siblings

Allow independent adjustment of minimum length for spanner siblings

The property minimum-length does not allow independent adjustment
of the siblings of broken spanners.  This patch adds a property,
minimum-length-after-break, which, if set, behaves similarly to minimum-length,
but only affects siblings starting a line.  If minimum-length-after-break is
unset, minimum-length still controls all lengths.

A regtest demonstrates usage of the new property with a number of spanners.
Another regtest demonstrates usage and compatibility of minimum-distance
and minimum-distance-after-break.