Ladybird: Allow right clicking and inspecting elements

This adds "Inspect Element" (currently the only entry) to the context
menu for the page, which will do what you expect (most of the time),
and bring up the Inspector with hovered element selected.

Ladybird/BrowserWindow.h

@@ -49,6 +49,7 @@ public slots:
     void reset_zoom();
     void select_all();
     void copy_selected_text();
+    void show_context_menu(QPoint const&);
 
 protected:
     bool eventFilter(QObject* obj, QEvent* event) override;