LibWeb: Move is_focusable() virtual from Element to EventTarget

Userland/Libraries/LibWeb/DOM/Element.h

@@ -103,8 +103,6 @@ public:
     ExceptionOr<void> set_inner_html(String const&);
 
     bool is_focused() const;
-    virtual bool is_focusable() const { return false; }
-
     bool is_active() const;
 
     NonnullRefPtr<HTMLCollection> get_elements_by_class_name(FlyString const&);

Userland/Libraries/LibWeb/DOM/EventTarget.h

@@ -26,6 +26,8 @@ public:
     void ref() { ref_event_target(); }
     void unref() { unref_event_target(); }
 
+    virtual bool is_focusable() const { return false; }
+
     void add_event_listener(const FlyString& event_name, RefPtr<EventListener>);
     void remove_event_listener(const FlyString& event_name, RefPtr<EventListener>);