Prohibit reversions by default, make allowlist for real reversions

[corneliusroemer]

As byproduct of synthetic Pango, output all detected reversions

As a second step: only allow reversions in a whitelist - requires review of potentially new true reversions

[corneliusroemer]

Reversions are one of the things listed in https://github.com/corneliusroemer/pango-sequences

The second step (Whitelist) is not yet implemented but is a good idea to reduce number of overwrites necessary.