Client: Set unconfigured xdg surfaces (v6) as not exposed

Previously, isExposed would always return true, which resulted in a
wl_egl_window being created and an illegal attach and commit occurring.
i.e. protocol errors such as:

zxdg_surface_v6@15: error 3: xdg_surface has never been configured

Change-Id: I277c65d663cfed0fe436a128fe1963d138f01c87
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
Reviewed-by: David Edmundson <davidedmundson@kde.org>

src/plugins/platforms/wayland/plugins/shellintegration/xdg-shell-v6/qwaylandxdgshellv6_p.h

@@ -86,6 +86,7 @@ public:
     void setAppId(const QString &appId) override;
 
     void setType(Qt::WindowType type, QWaylandWindow *transientParent) override;
+    bool isExposed() const override { return m_configured; }
     bool handleExpose(const QRegion &) override;
     bool handlesActiveState() const { return m_toplevel; }
     void applyConfigure() override;