Layouting: Avoid a lambda in the demo

Change-Id: Ic11936fd2a6da2153a52e39249d49d174ffb8466
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

tests/manual/layoutbuilder/comparison/layoutbuilder/mainwindow.h

@@ -18,7 +18,7 @@ Application app
 
         PushButton {
             text("Quit"),
-            onClicked([] { QApplication::quit(); })
+            onClicked(QApplication::quit)
         }
     }
 };

tests/manual/layoutbuilder/demo/main.cpp

@@ -31,7 +31,7 @@ int main(int argc, char *argv[])
 
                 PushButton {
                     text("Quit"),
-                    onClicked([] { QApplication::quit(); })
+                    onClicked(QApplication::quit)
                 },
              }
         }