closes #4: Favorite buttons has been extended with category buttons.

shop/data/favorititem.h

@@ -30,13 +30,13 @@ public:
 
     // IShopItem interface
 public:
-    long id() override;
+    long id() const override;
     void setId(long id);
 
     QString name() override;
     void setName(const QString &name);
 
-    virtual QString shortName() override;
+    QString shortName() override;
     void setShortName(const QString &shortName);
 
     QDecDouble unitPrice() override;
@@ -45,7 +45,7 @@ public:
     Enums::VatType vatType() override;
     void setVatType(const Enums::VatType &vatType);
 
-    QString pluginId() override;
+    QString pluginId() const override;
     void setPluginId(const QString &pluginId);
 
     QString favButtonName() const;