Add dark style preference
We Initiatives#32 for adding dark style preference to GNOME applications.
At the time, we only implemented support for following the system setting for the GTK theme in Maps, as the map at the time was always light (the raster map).
But now we have the new vector map style with dark support, so I think it's time revisit this.
One option is to go with a common switcher, using the existing libadwaita button styles for this:
But one suggestion before was to do something with a bit more "oumph", such as depicting small previews of map thumbnails.
As a comparison, when we had aerial map support, we used to show live-updated thumbnails, like this:
Another question is if this should be placed in the hamburger menu, or in the layers menu (where we used to have the the map type switch (street, aerial) when that was available.