fix splitting of (route) relation member ways

instead of fully re-sorting the whole relation every time a member is split, perform a local operation: This works under the assumption that the relation is already sorted properly. The new way is inserted into the relation before or after the existing member, depending on how the old/new way connect to their neighboring members.

for cases where two ways form a loop, a additional look-ahead is implemented to disambiguate the order

data/core.yaml

@@ -499,6 +499,7 @@ en:
           one: "Split a feature."
           other: "Split {n} features."
       not_eligible: Lines can't be split at their beginning or end.
+      parent_incomplete: This line cannot be split because it is part of a larger relation which has only been partially loaded. Make sure that all ways connected to this way are present on the map.
       connected_to_hidden: This can't be split because it is connected to a hidden feature.
     restriction:
       annotation: