Some more refactoring, reuse existing code.

Reuse the core help engine in the settings pages. Do not setup the gui
help engine at all since any getter will do so. Split update documentation
into smaller pieces for better performance.

Reviewed-by: ck

src/plugins/help/helpplugin.h

@@ -96,7 +96,7 @@ public:
     void setFilesToRegister(const QStringList &files);
 
 public slots:
-    void pluginUpdateDocumentation();
+    void slotRegisterDocumentation();
     void handleHelpRequest(const QString &url);
 
 private slots:
@@ -134,7 +134,9 @@ private slots:
 
 private:
     void resetFilter();
-    bool updateDocumentation();
+    bool verifiyDocumentation();
+    bool registerDocumentation();
+    bool unregisterDocumentation(const QStringList &nameSpaces);
 
 private:
     QToolBar *createToolBar();