Feature Request: OSM plugin: Add more raster tile services

[Artoria2e5]

https://www.openstreetmap.org/ currently offers six "featured" raster tile providers on the web page. We have 3 of them:

• [x] Standard
• [x] CyclOSM
• [ ] OpenCycleMap – commercial via thunderforest, need API key, we can try a free plan
• [ ] Transport Map – commercial via thunderforest, need API key, we can try a free plan – VERY USEFUL for Ingress-on-a-Bus activity
• [ ] Tracestrack Topo – commercial via tracestack, need API key, we can try a free plan
  • A completely free alternative might be OpenTopoMap
• [x] Humanitarian

I believe expanding our selection of OSM tiles to all six might be useful. For me, in particular, the Transport one is most useful. I don't really know what's the advantage of OCM over CyclOSM.

[Artoria2e5]

Thunderforest and tracestack tie into #228. I think we should implement a key entry window.

[McBen]

a free transportation map alternative (osm data) https://www.xn--pnvkarte-m4a.de

As plugin: https://gist.github.com/McBen/30761c55a8e409969c1eaab62c136537

(Quickly hacked for a friend. I was too lazy to create a public post)

[Artoria2e5]

Awesome! Now I can really plan my UPV/UPC trips on a bus.