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/mercurial/authenticationdialog.cpp

@@ -24,7 +24,7 @@ AuthenticationDialog::AuthenticationDialog(const QString &username, const QStrin
 
     auto buttonBox = new QDialogButtonBox(QDialogButtonBox::Cancel|QDialogButtonBox::Ok);
 
-    using namespace Utils::Layouting;
+    using namespace Layouting;
 
     Column {
         Form {