prefs: move external filter tree onload method (which checks for inactive feeds, etc.) to FilterTree class

classes/pref/feeds.php

@@ -983,10 +983,6 @@ class Pref_Feeds extends Handler_Protected {
 							dijit.byId('feedTree').editCategory(bare_id, item);
 						}
 					</script>
-					<script type="dojo/method" event="onLoad" args="item">
-						dijit.byId('feedTree').checkInactiveFeeds();
-						dijit.byId('feedTree').checkErrorFeeds();
-					</script>
 				</div>
 			</div>
 		</div>

js/PrefFeedTree.js

@@ -5,6 +5,16 @@ define(["dojo/_base/declare", "dojo/dom-construct", "lib/CheckBoxTree", "dojo/_b
 	function (declare, domConstruct, checkBoxTree, array, cookie) {
 
 	return declare("fox.PrefFeedTree", lib.CheckBoxTree, {
+		postCreate: function() {
+			this.inherited(arguments);
+
+			const tmph = dojo.connect(this, 'onLoad', () => {
+				dojo.disconnect(tmph);
+
+				this.checkInactiveFeeds();
+				this.checkErrorFeeds();
+			});
+		},
 		// save state in localStorage instead of cookies
 		// reference: https://stackoverflow.com/a/27968996
 		_saveExpandedNodes: function(){