For loot json files, include alternate coordinates for treasures inside interiors

[dshepsis]

Since the actual coordinates of interior areas (i.e. inside of buildings and such) don't correspond to their exteriors, any loot or other objects inside interiors have their icons placed in strange and confusing locations. These are normally very low underground, but vertical height isn't shown on the leaflet map, so it often isn't clear which treasures are inside buildings and which are outside, or how to get to figure out which buildings treasures are actually in. To amend this, I think there should be an optional property called displayCoords or similar, which, if present, overwrite the coords property for the sake of placing an icon on the leaflet map. The displayCoords will be manually chosen to correspond with the location of the exterior of the building the item is in. Such items may have special markers to indicate that they are inside buildings.