qdoc: Use the title instead of plain name when listing groups

When listing group nodes, for example, with \annotatedlist command,
QDoc created a link with the plain name (the string that was passed
to \group command as parameter) as the link text.

This change makes QDoc use the group \title, if one exists.

Change-Id: I30b7f9eefc60c12f67b1e579bbc4c5cebe84474b
Task-number: QTBUG-45420
Reviewed-by: Martin Smith <martin.smith@digia.com>

src/tools/qdoc/node.cpp

@@ -149,7 +149,7 @@ QString Node::plainFullName(const Node* relative) const
  */
 QString Node::fullName(const Node* relative) const
 {
-    if (isDocumentNode())
+    if ((isDocumentNode() || isGroup()) && !title().isEmpty())
         return title();
     return plainFullName(relative);
 }