Ladybird: Add IPC call for creating a new child tab

This will be used for choosing a navigable that requires opening a new
tab or new window. Such as calls to window.open(), or specific WebDriver
calls.

Ladybird/Qt/WebContentView.h

@@ -77,6 +77,8 @@ public:
     };
     void update_palette(PaletteMode = PaletteMode::Default);
 
+    using ViewImplementation::client;
+
 signals:
     void urls_dropped(QList<QUrl> const&);