Disable help search widget while indexing documentation

If you try to search while indexing, in the best case you'd get
incomplete results (though with a warning that that is the case), and in
the worst case it crashes because of QTBUG-66816. So just disable the
search widget with info and progress indicator in that case.

Task-number: QTCREATORBUG-20295
Change-Id: Ibec602b1bc0e98f6fef029c6f10c0cdc5197294c
Reviewed-by: Robert Loehning <robert.loehning@qt.io>

src/plugins/help/searchwidget.h

@@ -33,10 +33,13 @@
 
 QT_BEGIN_NAMESPACE
 class QHelpSearchEngine;
+class QHelpSearchQueryWidget;
 class QHelpSearchResultWidget;
 class QUrl;
 QT_END_NAMESPACE
 
+namespace Utils { class ProgressIndicator; }
+
 namespace Help {
 namespace Internal {
 
@@ -93,6 +96,9 @@ private:
 
     QHelpSearchEngine *searchEngine;
     QHelpSearchResultWidget *resultWidget;
+    QHelpSearchQueryWidget *m_queryWidget;
+    QWidget *m_indexingDocumentationLabel;
+    Utils::ProgressIndicator *m_indexingIndicator;
 };
 
 } // namespace Internal