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.cpp

@@ -526,6 +526,13 @@ void TextDocumentLayout::setFolded(const QTextBlock &block, bool folded)
         userData(block)->setFolded(true);
     else if (TextBlockUserData *userData = testUserData(block))
         userData->setFolded(false);
+    else
+        return;
+
+    TextDocumentLayout *layout = qobject_cast<TextDocumentLayout *>(
+                block.document()->documentLayout());
+    if (layout)
+        emit layout->foldChanged(block.blockNumber(), folded);
 }
 
 void TextDocumentLayout::requestExtraAreaUpdate()