Refactoring of GPS-location (uses featureSource too now), factoring out state, add ReplaceGeometryAction and conflation example

assets/layers/gps_location/gps_location.json

@@ -0,0 +1,15 @@
+{
+  "id": "gps_location",
+  "description": "Meta layer showing the current location of the user",
+  "minzoom": 0,
+  "source": {
+    "osmTags": "user:location=yes"
+  },
+  "mapRendering": [
+    {
+      "icon": "crosshair:#00f",
+      "iconSize": "40,40,center",
+      "location": "point"
+    }
+  ]
+}