Add icon property parameter to SMW queries

[JeroenDeDauw]

{{#ask: [[Category: Cities]]
|? Coords
|? City marker icon
| format = leaflet
| icon property = City marker icon
}}

• Berlin: City marker icon=BlueMarker.png, Coords=whatever
• London: City marker icon=RedMarker.png, Coords=whatever
• New York: City marker icon=GreenMarker.png, Coords=whatever

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Inspiration: https://github.com/SemanticMediaWiki/SemanticResultFormats/blob/master/formats/filtered/src/View/MapView.php#L155