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/sidebysidediffeditorwidget.h

@@ -40,6 +40,7 @@ class TextEditorWidget;
 QT_BEGIN_NAMESPACE
 class QMenu;
 class QSplitter;
+class QTextBlock;
 QT_END_NAMESPACE
 
 namespace DiffEditor {