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/texteditor/texteditorplugin.cpp

@@ -56,6 +56,7 @@
 #include <QDir>
 
 using namespace Core;
+using namespace Utils;
 
 namespace TextEditor {
 namespace Internal {