Use the new KConfig WindowStateSaver

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

src/main.cpp

@@ -304,7 +304,6 @@ int main(int argc, char *argv[])
     QWindow *window = windowFromEngine(&engine);
 
     WindowController::instance().setWindow(window);
-    WindowController::instance().restoreGeometry();
 
     return app.exec();
 }