Add opentopomap.org as a map service to gnome-maps
Currently gnome-maps supports a street and aerial map service, provided by mapbox. When I import outdoor GPX/KMLs (mosly hiking/cycling routes), I find myself missing a topographic map from time to time. According to the OSM wiki opentopomap.org's tile service may be embedded into projects (as long as proper attribution is given).
Would the community appreciate opentopomap.org as an extra map service? Note that its style is quite different from mapbox's street style. Changes in gnome-maps would involve (at least) adding a new MapType, a new maps-service definition and updating layersPopOver.js. Initial testing indicates that the opentopomap tiling service is compatible with libchamberlain and renders successfully in gnome-maps (tiles are sized 256 though), so I don't anticipate major issues.