multi-page: prepare data for index generation
multi-page: prepare data for index generation

Using StreetIndex, we build an index for each page of the map, and
then with various sorting and grouping tricks, we merge those indexes,
categories by categories. This will allow to generate the index with
data that is properly sorted.

Signed-off-by: Thomas Petazzoni <a5d87d2bd7c424ee5cbdb4e978635bf653d5a005@enix.org>

maposmatic/ocitysmap,dittaeva/ocitysmap,maposmatic/ocitysmap,maposmatic/ocitysmap,dittaeva/ocitysmap