Raster layer browser

[martinRenou]

When implementing #9, a raster layer browser should be created. We could use https://github.com/geopandas/xyzservices/blob/main/provider_sources/xyzservices-providers.json to automatically populate this browser at build time. Ideally, it would look like the felt browser (see screenshot) and the pre-populated raster layer options would show a thumbnail (also generated at build time).

Of course, we should still provide the option to define a custom raster layer from a user-defined URL.

[brichet]

Closing as completed by #16