Utils: Make Layouting a top level namespace

The whole machinery is now almost only layoutbuilder.{h,cpp},
mostly independent of the rest of Utils. Idea is to finish the
separation to make it stand-alone usable also outside creator.

Change-Id: I958aa667d17ae26b21209f22412309c5307a579c
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/texteditor/behaviorsettingswidget.cpp

@@ -165,7 +165,7 @@ BehaviorSettingsWidget::BehaviorSettingsWidget(QWidget *parent)
 
     d->groupBoxMouse = new QGroupBox(Tr::tr("Mouse and Keyboard"));
 
-    using namespace Utils::Layouting;
+    using namespace Layouting;
 
     const auto indent = [](QWidget *inner) { return Row { Space(30), inner }; };