refactoring(maplibre): add RasterLayer info, add background switch, add default global layers

UI/Map/RasterLayerPicker.svelte

@@ -0,0 +1,18 @@
+<script lang="ts">
+  import type { Readable, Writable } from "svelte/store";
+  import type { RasterLayerPolygon } from "../../Models/RasterLayers";
+
+  /***
+   * Chooses a background-layer out of available options
+   */
+  export let availableLayers: Readable<RasterLayerPolygon[]>
+  export let value: Writable<RasterLayerPolygon>
+</script>
+
+<select bind:value={$value}>
+  {#each $availableLayers as availableLayer }
+    <option value={availableLayer}>
+      {availableLayer.properties.name}
+    </option>
+  {/each}
+</select>