Core/Utils: Migrate further to Utils::Id

The coreplugin/id.h header is kept for downstream for now.

Change-Id: I8c44590f7b988b3770ecdc177c40783e12353e66
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/navigationwidget.cpp

@@ -48,6 +48,8 @@
 
 Q_DECLARE_METATYPE(Core::INavigationWidgetFactory *)
 
+using namespace Utils;
+
 namespace Core {
 
 NavigationWidgetPlaceHolder *NavigationWidgetPlaceHolder::s_currentLeft = nullptr;