Hide experimental map when unavailable

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

Previously, the "Enable Experimental Map" toggle would be greyed out if libshumate was not built with vector tile support, but now it is hidden entirely. Fixes #591 (closed).

Also, fix a crash if the map source is set to vector but libshumate is built without vector renderer support.

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