Declaratively set plugin card list scroll handler

src/gui/html/index.html

@@ -317,7 +317,7 @@
             <!-- Generated message <li> elements go here. -->
           </ul>
         </paper-material>
-        <iron-list id="pluginCardList" as="plugin">
+        <iron-list id="pluginCardList" as="plugin" scroll-target="main">
           <template>
             <loot-plugin-card id="[[plugin.id]]"
                               data="[[plugin]]">

src/gui/html/js/init.js

@@ -233,8 +233,6 @@
 
   return () => {
     Dialog.showProgress('Initialising user interface...');
-    /* Set the plugin list's scroll target to its parent. */
-    document.getElementById('pluginCardList').scrollTarget = document.getElementById('main');
 
     /* Make sure settings are what I want. */
     window.marked.setOptions({