Merge: Promote location in MEntity

For some unknown historical reasons, the location was only provided to some specific model entities. By extending it to all entities, this simplify clients that need to process, visit or filter entities.

Location is not nullable for all entities. This is simpler but need some place-holder object for fictive location. ie some location objects that mean "no location".

Pull-Request: #2045
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>