Utils: Avoid intermediate widgets when using LayoutBuilder

In most cases, the layout constructed in the builder was set
on a widget which in turn was put into a vbox in the actual
widget. This is not necessary, but needs some re-ordering.

Also make sure that using not-yet-parented widgets during
layout construction does not cause visible artifacts.

Change-Id: I75727a571da093d3131ea6fba467c2c646cdb6f1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/valgrind/valgrindconfigwidget.cpp

@@ -88,14 +88,14 @@ ValgrindConfigWidget::ValgrindConfigWidget(ValgrindBaseSettings *settings)
         s.minimumInclusiveCostRatio, nl,
         s.visualizationMinimumInclusiveCostRatio, nl,
         s.enableEventToolTips, nl,
-        Item {
+        Span {
+            2,
             Group {
                 s.enableCacheSim,
                 s.enableBranchSim,
                 s.collectSystime,
                 s.collectBusEvents,
-            },
-            2 // Span.
+            }
         }
     };