I.e. horizontally or "breadth first" rather than vertical tree traversal.
This allows to make sure all the start pages in particular are searched,
when using max_page option.
With the new case_mapping lang property, we can consider upper and lower
case versions of the same character as one character.
This makes sense in some language, and would allow to enter some rarer
characters (but still in the main alphabet) inside the frequent
character list. For instance 'œ' and 'Œ' in French.
