[mod] simple theme, JS: introduce window.searxng.theme namespace

In window.searxng.theme the theme data can be passed through to JS
implementations.

Initial the window.searxng.theme namespace starts with a value for
`img_load_error`::

    // image that is displayed if load of <img src='...'> failed
    img_load_error: 'img/img_load_error.svg'

The searx/static/themes/__common__/js/image_layout.js is patched to uses the
value, if the theme defines a value for img_load_error in this namespace.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/__common__/js/image_layout.js

@@ -149,7 +149,7 @@
 
     function img_load_error(event) {
       // console.log("ERROR can't load: " + event.originalTarget.src);
-      event.originalTarget.src = window.searxng.static_path + 'img/img_load_error.svg';
+      event.originalTarget.src = w.searxng.static_path + w.searxng.theme.img_load_error;
     }
 
     function throttleAlign() {
@@ -175,7 +175,9 @@
         img.addEventListener('load', throttleAlign);
         // https://developer.mozilla.org/en-US/docs/Web/API/GlobalEventHandlers/onerror
         img.addEventListener('error', throttleAlign);
-        img.addEventListener('error', img_load_error, {once: true});
+        if (w.searxng.theme.img_load_error) {
+          img.addEventListener('error', img_load_error, {once: true});
+        }
       }
     }
   };

searx/static/themes/simple/src/js/head/00_init.js

@@ -24,6 +24,10 @@
         hotkeys: script.getAttribute('data-hotkeys') === 'true',
         static_path: script.getAttribute('data-static-path'),
         translations: JSON.parse(script.getAttribute('data-translations')),
+        theme : {
+            // image that is displayed if load of <img src='...'> failed
+            img_load_error: 'img/img_load_error.svg'
+        }
     };
 
     // update the css