Use the new KConfig WindowStateSaver

This removes some NeoChat-specific code we have for saving/restoring the
window.

src/windowcontroller.h

@@ -41,16 +41,6 @@ public:
      */
     QWindow *window() const;
 
-    /**
-     * @brief Restore any saved window geometry if available.
-     */
-    void restoreGeometry();
-
-    /**
-     * @brief Save the current window geometry.
-     */
-    Q_INVOKABLE void saveGeometry();
-
     /**
      * @brief Show the window and raise to the top.
      */