Fix broken behavior for welcome page links if no docs are installed.

In case of split mode we did not check if the actual requested doc
exists and thus did fail to open the browser if it could not be found.

Reviewed-by: Daniel Molkentin

src/plugins/help/helpmanager.h

@@ -47,12 +47,11 @@ class HELP_EXPORT HelpManager : public QObject
 public:
     HelpManager(Internal::HelpPlugin*);
 
-    void openHelpPage(const QString& url);
-    void openContextHelpPage(const QString &url);
+    void handleHelpRequest(const QString &url);
     void registerDocumentation(const QStringList &fileNames);
 
 signals:
-    void helpPluginUpdateDocumentation();
+    void registerDocumentation();
 
 private:
     Internal::HelpPlugin *m_plugin;