Actually hide gps-track, add better documentation output

Models/ThemeConfig/LayerConfig.ts

@@ -424,8 +424,14 @@ export default class LayerConfig extends WithContextLoader {
             if (addedByDefault) {
                 extraProps.push("**This layer is included automatically in every theme. This layer might contain no points**")
             }
+            if(this.shownByDefault === false){
+                extraProps.push('This layer is not visible by default and must be enabled in the filter by the user. ')
+            }
             if (this.title === undefined) {
-                extraProps.push("Not clickable by default. If you import this layer in your theme, override `title` to make this clickable")
+                extraProps.push("This layer cannot be toggled in the filter view. If you import this layer in your theme, override `title` to make this toggleable.")
+            }
+            if(this.title === undefined && this.shownByDefault === false){
+                extraProps.push("This layer is not visible by default and the visibility cannot be toggled, effectively resulting in a fully hidden layer. This can be useful, e.g. to calculate some metatags. If you want to render this layer (e.g. for debugging), enable it by setting the URL-parameter layer-<id>=true")
             }
             if (this.name === undefined) {
                 extraProps.push("Not visible in the layer selection by default. If you want to make this layer toggable, override `name`")

assets/layers/gps_location_history/gps_location_history.json

@@ -7,6 +7,5 @@
     "#": "Cache is disabled here as these points are kept seperately",
     "maxCacheAge": 0
   },
-  "mapRendering": null,
+  "mapRendering": null
-  "shownByDefault": false
 }

assets/layers/gps_track/gps_track.json

@@ -9,6 +9,7 @@
   "title": {
     "render": "Your travelled path"
   },
+  "shownByDefault": false,
   "tagRenderings": [
     {
       "id": "Privacy notice",