Refactor QtHelp API according to deprecation of QMap as a multi map

As of Qt 5.15 the usage of QMap as a multi map is deprecated. Currently QtHelp module exposes some API that use QMap as a multi map:

QMap<QString, QUrl> QHelpEngineCore::linksForIdentifier(const QString &id) const;
QMap<QString, QUrl> QHelpEngineCore::linksForKeyword(const QString &keyword) const;

QMap<QString, QUrl> QHelpIndexModel::linksForKeyword(const QString &keyword) const;
[signal] void QHelpIndexWidget::linkActivated(const QUrl &link, const QString &keyword);
[signal] void QHelpIndexWidget::linksActivated(const QMap<QString, QUrl> &links, const QString &keyword);

So, we need to deprecate the above methods and provide replacement. However, changing the data structure will cause a conflict inside QHelpEngineCore, since we can't return a different type for linksForIdentifier(const QString &id). So we need to change names in QHelpEngineCore for the mentioned methods.

Possible solutions:
1) Replace QMap<QString, QUrl> with QMultiMap<QString, QUrl>.
2) Replace QMap<QString, QUrl> with QList<QHelpSearchResult>. However, this struct contains the "snippet" field, which would be useless in this case.
3) Provide a new struct, e.g. QHelpLink (name to be discussed), containing QUrl and QString (as a doc title) and replace QMap<QString, QUrl> with QList<QHelpLink>.

...Other options?

My preference is: 3 (the best option), 2 and then 1 (the worst).

Note, that we should still do it for Qt 5.15.