Add folding to Side-by-side diff editor

Add folding for files and chunks.

Change-Id: I76476351e88f0b3e71e3cccbca0fa17b02c26226
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/diffeditor/selectabletexteditorwidget.h

@@ -50,6 +50,8 @@ public:
     ~SelectableTextEditorWidget() override;
     void setSelections(const QMap<int, QList<DiffSelection> > &selections);
 
+    static void setFoldingIndent(const QTextBlock &block, int indent);
+
 private:
     void paintBlock(QPainter *painter,
                     const QTextBlock &block,