Refactor communication between map and assets

[m-mohr]

While fixing #312 it shows that once we add more file formats than just GeoTiff + JSON, the communication between the Maps and Assets component must get more clear. It must be strictly divided into the basic STAC data (e.g. bbox/geometry) and additional data such as item bboxes and the GeoTiffs or a JSON file.

Also there's currently no way to get the item bboxes back once another asset is shown.