DiffEditor: add info bar for documents with encoding errors

... with the option to reload the diff document with another encoding.

Fixes: QTCREATORBUG-23835
Change-Id: I1ce07b292688059b37535f2972970d8ea91be81b
Reviewed-by: hjk <hjk@qt.io>

src/plugins/diffeditor/diffeditordocument.h

@@ -86,6 +86,7 @@ public:
     bool reload(QString *errorString, ReloadFlag flag, ChangeType type) override;
     OpenResult open(QString *errorString, const QString &fileName,
                     const QString &realFileName) override;
+    bool selectEncoding();
     State state() const { return m_state; }
 
     QString plainText() const;