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/cppeditor/cppeditorwidget.cpp

@@ -105,6 +105,7 @@ using namespace Core;
 using namespace CPlusPlus;
 using namespace CppTools;
 using namespace TextEditor;
+using namespace Utils;
 
 namespace CppEditor {
 namespace Internal {