Add overlay layer possibility, fix #515

Models/ThemeConfig/Json/LayoutConfigJson.ts

@@ -1,5 +1,7 @@
 import {TagRenderingConfigJson} from "./TagRenderingConfigJson";
 import {LayerConfigJson} from "./LayerConfigJson";
+import TilesourceConfig from "../TilesourceConfig";
+import TilesourceConfigJson from "./TilesourceConfigJson";
 
 /**
  * Defines the entire theme.
@@ -155,6 +157,11 @@ export interface LayoutConfigJson {
      */
     defaultBackgroundId?: string;
 
+    /**
+     * Define some (overlay) slippy map tilesources
+     */
+    tileLayerSources?: TilesourceConfigJson[]
+
     /**
      * The number of seconds that a feature is allowed to stay in the cache.
      * The caching flow is as following:

Models/ThemeConfig/Json/TilesourceConfigJson.ts

@@ -0,0 +1,36 @@
+/**
+ * Configuration for a tilesource config
+ */
+export default interface TilesourceConfigJson {
+
+
+    /**
+     * The path, where {x}, {y} and {z} will be substituted
+     */
+    source: string,
+    
+    isOverlay?: boolean,
+
+    /**
+     * How this will be shown in the selection menu.
+     * Make undefined if this may not be toggled
+     */
+    name?: any | string
+
+    /**
+     * Only visible at this or a higher zoom level
+     */
+    minZoom?: number
+
+    /**
+     * Only visible at this or a lower zoom level
+     */
+    maxZoom?: number
+
+
+    /**
+     * The default state, set to false to hide by default
+     */
+    defaultState: boolean;
+
+}