Core: Make IEditor::id a value member and adjust users

Change-Id: I47e457c4a1821c515cd8a740d27bfe005a0c09e6
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/diffeditor/diffeditor.h

@@ -67,7 +67,6 @@ public:
 
     bool open(QString *errorString, const QString &fileName, const QString &realFileName);
     Core::IDocument *document();
-    Core::Id id() const;
 
     QWidget *toolBar();