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Abstract Marker class for further extensions. This class handles the caption of a marker.
Holds a collection of markers. Marker could mark a POI (e.g. restaurants)
or ways (e.g. special interest areas). Use this class if you often
access the markers by their item. This is the case if you want to change
the rendering of the marker.
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Provides a grid for the given bounding box and stores each marker in its respective cell. When retrieving the markers by cells the user
can decide to show the marker itself or a cluster-marker instead of several markers of the same cell.