Added icons to options dialog categories

Some categories do not have icons yet, unfortunately.

src/plugins/help/docsettingspage.h

@@ -48,6 +48,7 @@ public:
     QString displayName() const;
     QString category() const;
     QString displayCategory() const;
+    QIcon categoryIcon() const;
 
     QWidget *createPage(QWidget *parent);
     void apply();