WebUI: Filter list improvements

A couple of tweaks to make them a little bit better:
1. Make highlighting functions more consistent (this also fixes minuscule bug when no filter item in tracker list is highlighted due to a type mismatch)
2. Use [event delegation](https://javascript.info/event-delegation) to handle filter toggling & item selection
3. Other minor improvements (everything should work like it was previously)

PR #21191.

src/webui/www/private/scripts/contextmenu.js

@@ -179,6 +179,10 @@ window.qBittorrent.ContextMenu ??= (() => {
             this.setupEventListeners(t);
         },
 
+        searchAndAddTargets: function() {
+            document.querySelectorAll(this.options.targets).forEach((target) => { this.addTarget(target); });
+        },
+
         triggerMenu: function(e, el) {
             if (this.options.disabled)
                 return;