Added icons to options dialog categories

Some categories do not have icons yet, unfortunately.

src/plugins/help/generalsettingspage.h

@@ -46,9 +46,10 @@ public:
     GeneralSettingsPage();
 
     QString id() const;
-    virtual QString displayName() const;
+    QString displayName() const;
     QString category() const;
     QString displayCategory() const;
+    QIcon categoryIcon() const;
 
     QWidget *createPage(QWidget *parent);
     void apply();