Improve common tree segments renaming

[pviscone]

Currently all the Tree segments that are shared among multiple regions are renamed ($n)common.
This make it difficult to recognize them. Preserve the name of the segment.

Technically the Tree cut and glue algorithm requires a serious reformatting, probably also changing the underling data structure but it would require some effort.

[pviscone]

Segments' name wrapped into another list and passed to clean_common as quick workaround in b81f1ac.

Everything should be wrapped inside a proper data structure