Update current location marker

[seadowg]

Closes #7106

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[seadowg]

I've marked this as "needs testing" as it's at a good point to have a play with and see if we can find bugs before I finish off. The blue dot should now show in all map views (without the "accuracy halo" in OSM/Mapbox as before), but the main thing to investigate is the maps and placement-map appearances as I had to do the most rework in that area of the code.