Help index filter optimizations

Move retrieval away from prepare search, and do some caching. Also split
the keyword search into individual chunks per help database that are
executed on the UI thread individually.

Change-Id: I0b8fe4abfc3cba46620985752d3d90638e10512f
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/coreplugin/helpmanager.h

@@ -63,7 +63,7 @@ public:
 
     typedef QHash<QString, QStringList> Filters;
 
-    static QObject *instance();
+    static HelpManager *instance();
     static QString collectionFilePath();
 
     static void registerDocumentation(const QStringList &fileNames);
@@ -74,9 +74,6 @@ public:
 
     static QMap<QString, QUrl> linksForKeyword(const QString &key);
     static QMap<QString, QUrl> linksForIdentifier(const QString &id);
-    static QStringList findKeywords(const QString &key,
-                             Qt::CaseSensitivity caseSensitivity = Qt::CaseInsensitive,
-                             int maxHits = INT_MAX);
 
     static QUrl findFile(const QUrl &url);
     static QByteArray fileData(const QUrl &url);