QTextMarkdownImporter: Fix performance for non-trivial documents

While parsing the markdown document, each addition of text and
formats was triggering a relayout of the so far existing document,
leading to quadratic behavior and very bad performance for any
non-trivial markdown document.

Guard the changes while parsing with a begin/endEditBlock to
avoid these intermediate updates.

The performance impact can for example be observed with the
markdown editor in Qt Creator (11+).

Pick-to: 6.5
Change-Id: I5f89441ea41bc3c6281b616f0c5528b49b48e432
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/text/qtextmarkdownimporter.cpp

@@ -138,7 +138,9 @@ void QTextMarkdownImporter::import(QTextDocument *doc, const QString &markdown)
         m_monoFont.setPixelSize(doc->defaultFont().pixelSize());
     qCDebug(lcMD) << "default font" << doc->defaultFont() << "mono font" << m_monoFont;
     QByteArray md = markdown.toUtf8();
+    m_cursor->beginEditBlock();
     md_parse(md.constData(), MD_SIZE(md.size()), &callbacks, this);
+    m_cursor->endEditBlock();
     delete m_cursor;
     m_cursor = nullptr;
 }