Add layer opacity

As requested by @Bfritz0815

config/rzl.js

@@ -398,12 +398,14 @@ const config : Config = {
       image: require("../img/layers/rzl/rooms.svg"),
       baseLayer: true,
       name: "RaumZeitLabor",
-      defaultVisibility: "visible"
+      defaultVisibility: "visible",
+      opacity: 0.7
     },
     {
       image: require("../img/layers/rzl/details.svg"),
       name: "Details",
-      defaultVisibility: "visible"
+      defaultVisibility: "visible",
+      opacity: 0.4
     },
     {
       image: require("../img/layers/rzl/labels.svg"),

src/map.js

@@ -63,7 +63,7 @@ class SpaceMap extends React.Component<{state: State, width: number, height: num
     return (
       <LayersControlType name={layer.name}
             checked={layer.defaultVisibility == "visible"}>
-        <ImageOverlay url={layer.image} bounds={bounds} />
+        <ImageOverlay url={layer.image} bounds={bounds} opacity={layer.opacity} />
       </LayersControlType>
     );
   }

types/types.js

@@ -75,6 +75,7 @@ declare type Layer = {
   name: string,
   baseLayer: boolean,
   defaultVisibility: "visible" | "hidden",
+  opacity: number
 };
 
 declare type StateAction = {