Fix QDeclarativeGeoMap::populateMap duplicating items

Since apparently children() and childItems() do not necessarily return
disjoint sets, concatenating the two lists did, in some cases, cause
duplicated items in the map.

This patch resorts to uniting sets to remove the duplicates.

Change-Id: I07ef19a2fdff65429eb65d92be278d7c02ac1999
Reviewed-by: BogDan Vatra <bogdan@kdab.com>