Manage layers?

[severo]

In the future, if we allow more types of image background (OSM, or directly all leaflet base maps?, videos), and more processing options (voronoi, delauney, automatic detection, isochrone, etc.), we might need to display these options as "layers", as it's traditionnally done in mapping (or image edition) applications, eg.
https://geobretagne.fr/mapfishapp/map/1594459bdb8784466c25c27cfb3a6694

Note that it might add some complexity to the code.