x11/panelwindow: use Qt window default screen if none is provided

Fixes panels not updating geometry or attachments under X if a screen
was not explicitly provided by the user.

src/x11/panel_window.hpp

@@ -92,7 +92,7 @@ private slots:
 	void onScreenVirtualGeometryChanged();
 
 private:
-	void connectScreen();
+	void updateScreen();
 	void updateStrut(bool propagate = true);
 	void updateStrutCb() { this->updateStrut(); }
 	void updateAboveWindows();