Make Toplevel::window() virtual and implement in ShellClient

We need a shared window id in some way to e.g. properly support TabBox.
Though I do not like that the return value is xcb_window_t. This should
be either changed or a new "generic" windowId needs to be introduced.

shell_client.cpp

@@ -386,4 +386,9 @@ bool ShellClient::isInternal() const
            m_shellSurface->client() == waylandServer()->qtConnection();
 }
 
+xcb_window_t ShellClient::window() const
+{
+    return windowId();
+}
+
 }

shell_client.h

@@ -86,6 +86,7 @@ public:
     bool userCanSetFullScreen() const override;
     bool userCanSetNoBorder() const override;
     bool wantsInput() const override;
+    xcb_window_t window() const override;
 
     quint32 windowId() const {
         return m_windowId;

toplevel.h

@@ -205,7 +205,7 @@ class Toplevel
 public:
     explicit Toplevel();
     virtual xcb_window_t frameId() const;
-    xcb_window_t window() const;
+    virtual xcb_window_t window() const;
     QRect geometry() const;
     QSize size() const;
     QPoint pos() const;