Move a part of the index pages related code to ProofreadIndexPage

Clean also a lot of old code and introduce some unit tests

The move of the management of the pr_index table in a specific class will
be done in an other commit

This commit add also the support of navigators with a bad unicode support
and fix an issue with EditPage API.

Change-Id: Ibf19db7b691056d9e211e2cab7750086f4ae8abd