WIP: Add a spatial datastructure for Map markers.

In its current form, this is just a number of bins with ~10km resolution
containing the images instead of the list of images that we previously
had.
The new structure is only a building block that can allow us to:

• skip entire bins instead of individual images
• represent one bin with a single marker instead of one per image
• build a GeoCluster tree-like structure on top of the GeoBins, allowing

us to do a level-of-detail rendering merging different bins together