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/tabsettingswidget.cpp

@@ -87,7 +87,7 @@ TabSettingsWidget::TabSettingsWidget(QWidget *parent) :
     tabSizeLabel->setBuddy(m_tabSize);
     indentSizeLabel->setBuddy(m_indentSize);
 
-    using namespace Utils::Layouting;
+    using namespace Layouting;
     const auto indent = [](QWidget *inner) { return Row { Space(30), inner }; };
 
     Column {