Help/litehtml: Implement text search

Build an index between (visible) text and litehtml elements, so we can
use the regular text search functionality of Qt.

Change-Id: I2bf29628482f81c67b20b9a02d7808a330a073eb
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/help/litehtmlhelpviewer.cpp

@@ -198,8 +198,10 @@ void LiteHtmlHelpViewer::addForwardHistoryItems(QMenu *forwardMenu)
 bool LiteHtmlHelpViewer::findText(
     const QString &text, Core::FindFlags flags, bool incremental, bool fromSearch, bool *wrapped)
 {
-    // TODO
-    return false;
+    return m_viewer->findText(text,
+                              Core::textDocumentFlagsForFindFlags(flags),
+                              incremental,
+                              wrapped);
 }
 
 void LiteHtmlHelpViewer::copy()