Fix Help index filter after recent refactoring

It was incorrectly updating cached results, so e.g. shortening the
search string was not forcing a new search.

Change-Id: Ie55fce72b7363258195423f5686f013aeafc7d57
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/help/helpindexfilter.cpp

@@ -129,6 +129,7 @@ QList<LocatorFilterEntry> HelpIndexFilter::matchesFor(QFutureInterface<LocatorFi
     Utils::sort(keywords);
     keywords << unsortedKeywords;
     m_keywordCache = allresults;
+    m_searchTermCache = entry;
     foreach (const QString &keyword, keywords)
         entries.append(LocatorFilterEntry(this, keyword, QVariant(), m_icon));