Replace var with let in examples (#484)

* Replace var with let in examples

store-collected-images/webextension-plain/deps/uuidv4.js

* Reverted third–party code

menu-remove-element/background.js

@@ -1,6 +1,6 @@
 "use strict";
 
-var popupParameters;
+let popupParameters;
 
 browser.menus.create({
   id: "remove_element",

menu-remove-element/contentscript.js

@@ -52,7 +52,7 @@
   });
   
   
-  var highlightedBox;
+  let highlightedBox;
   function highlightElement(element) {
     removeHighlights();
     let boundingRect = element.getBoundingClientRect();

menu-remove-element/menusGetTargetElementPolyfill.js

@@ -18,7 +18,7 @@
 //   if the given first parameter matches the info.targetElementId integer from
 //   the menus.onClicked or menus.onShown events.
 if (!browser.menus || !browser.menus.getTargetElement) {
-  var menuTarget = null;
+  let menuTarget = null;
   let cleanupIfNeeded = () => {
     if (menuTarget && !document.contains(menuTarget)) menuTarget = null;
   };