TextEditor: fix folding of all nested blocks of folded block

Now all sub-blocks of folded block don`t became folded too when we add
new blocks in document

Change-Id: Id347d99195d2835a1de46a397175ff9fa68bb1ae
Reviewed-by: David Schulz <david.schulz@theqtcompany.com>

src/plugins/texteditor/textdocumentlayout.cpp

@@ -659,6 +659,7 @@ void TextDocumentLayout::FoldValidator::process(QTextBlock block)
             && !TextDocumentLayout::canFold(previous))
             || (!TextDocumentLayout::isFolded(previous)
                 && TextDocumentLayout::canFold(previous)
+                && previous.isVisible()
                 && !block.isVisible())) {
         TextDocumentLayout::setFolded(previous, !TextDocumentLayout::isFolded(previous));
     }