Showing applied extended statistics in explain Part 2

Hi,

This original patch made by Tomas improves the usability of extended statistics,
so I rebased it on 362de947, and I'd like to re-start developing it.

The previous thread [1] suggested something to solve. I'll try to solve it as
best I can, but I think this feature is worth it with some limitations.
Please find the attached file.

[1] https://www.postgresql.org/message-id/flat/8081617b-d80f-ae2b-b79f-ea7e926f9fcf%40enterprisedb.com

Regards,
Tatsuro Yamada
NTT Open Source Software Center