Move Id from Core to Utils

And add a compatibility wrapper for Core::Id, so we don't have to rename
all occurrences from Core::Id to Utils::Id.

This allows us to use Id also in Utils, which makes it possible to e.g.
move Core::InfoBar to Utils without work arounds.

Change-Id: I5555d05b4e52f09d501dbfe5d91252a982a97c61
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/texteditor/snippets/snippet.h

@@ -27,12 +27,12 @@
 
 #include <texteditor/texteditor_global.h>
 
+#include <coreplugin/id.h>
+
 #include <QChar>
 #include <QList>
 #include <QString>
 
-namespace Core { class Id; }
-
 namespace TextEditor {
 
 class TEXTEDITOR_EXPORT NameMangler