Add experimental vector layer

James Westman requested to merge jwestman/gnome-maps:vector-1 into main

Adds a toggle in the Layers popover, "Enable experimental map", which enables a vector layer using GNOME's tile server and the OSM Liberty stylesheet. The toggle only appears if libshumate is built with vector tile support.

I haven't implemented symbol-clicked yet because that will require changes to the place store (since the tiles don't contain persistent IDs).

Also fixed bugs in the layer switching code where the new layer was added on top of marker layers and the old layer was not removed.

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