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/texteditor/textdocumentlayout.h

@@ -219,6 +219,7 @@ public:
 
 signals:
     void updateExtraArea();
+    void foldChanged(const int blockNumber, bool folded);
 };
 
 } // namespace TextEditor