Ladybird: Move the page context menu from the BrowserWindow to the Tab

This will allow us to show different context menus depending on what
element is clicked, much like we do for Browser on Serenity.

Ladybird/Tab.h

@@ -67,6 +67,8 @@ private:
     QString m_title;
     QLabel* m_hover_label { nullptr };
 
+    OwnPtr<QMenu> m_page_context_menu;
+
     int tab_index();
 
     bool m_is_history_navigation { false };