Support of classes with unstructured location property

This issue is about making it possible to create a map based on a class containing a configurable property such as "location" which represents an address in an unstructured manner, for instance: "Park Avenue, New York". A use case of this feature is the representation of all XWikiUsers registered on a wiki on a map.

Several implementation paths are possible:

• Compute all the geolocations at rendering time, but in case of a large number of users, this can delay significantly the display of the map.
• Compute the geolocations only upon a specific action (such as a "recompute geolocations" buttons), which would then store the geolocations as PointClass objects for instance attached to the map page (since we cannot assume that the pages containing the objects holding the addresses are writable). Or, possibly better, consider that the "recompute geolocations" action is available only to an administrator, and add the compted PointClass objects directly to the pages they relate to.