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/diffeditor/selectabletexteditorwidget.cpp

@@ -33,7 +33,7 @@
 namespace DiffEditor {
 namespace Internal {
 
-SelectableTextEditorWidget::SelectableTextEditorWidget(Core::Id id, QWidget *parent)
+SelectableTextEditorWidget::SelectableTextEditorWidget(Utils::Id id, QWidget *parent)
     : TextEditorWidget(parent)
 {
     setFrameStyle(QFrame::NoFrame);