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/coreplugin/dialogs/saveitemsdialog.cpp

@@ -59,7 +59,7 @@ SaveItemsDialog::SaveItemsDialog(QWidget *parent, const QList<IDocument *> &item
 
     m_saveBeforeBuildCheckBox->setVisible(false);
 
-    using namespace Utils::Layouting;
+    using namespace Layouting;
     // clang-format off
     Column {
         m_msgLabel,