Merge basemaps, overlays and datasets

Having those three sections is a bit confusing. It would be nice to merge the three sections into a generic Datasets sections, and having the possibility for all datasets:

• show/hide,
• change opacity (some datasets may not support it, ideally the UI should then be disabled)
• change ordering of map layers (and being clear about 3D objects being unaffected)
• enable clipping & grid
• delete (potentially "breaking" the app experience if we delete all elevation layers or all color layers)

Maybe still distinguish from pure 2D map layers and from 3D objects to make sure users understand about ordering, masking, etc.

From a technical point of view, I think this is mandatory for #69 (closed), as we'd need to add overlays & stuff after the app is initialized.