Allow side-loading of offline maps

[pflarue]

It would be nice if Graywolf would recognize (perhaps only at restart) and use a .pmtiles file downloaded for another installation of graywolf that has been copied into the tiles directory.

This would allow the maps downloaded by one offline installation to be shared with other offline installations without having to reconnect to the Internet.

[ropucyka]

Columba just has an "Import" button for offline maps (https://wiki.openstreetmap.org/wiki/MBTiles).
I'm not sure if this is the best approach, but it could turn this into a fully offline radio app.