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/todo/todoprojectsettingswidget.cpp

@@ -32,7 +32,7 @@ TodoProjectSettingsWidget::TodoProjectSettingsWidget(ProjectExplorer::Project *p
 
     auto addExcludedPatternButton = new QPushButton(Tr::tr("Add"));
 
-    using namespace Utils::Layouting;
+    using namespace Layouting;
 
     Column {
         Group {