Added sell commodity feature.

shop/data/voucheritem.cpp

@@ -39,8 +39,9 @@ int VoucherItem::count() const
 
 void VoucherItem::setCount(int count)
 {
+    int oldCount = m_count;
     m_count = count;
-    emit countChanged();
+    emit countChanged(oldCount);
 }
 
 QDecDouble VoucherItem::unitPrice() const

shop/data/voucheritem.h

@@ -63,7 +63,7 @@ public:
     void setVoucher(const QWeakPointer<Voucher> &voucher);
 
 signals:
-    void countChanged();
+    void countChanged(int oldCount);
 
 private:
     friend class odb::access;