Revert "Remove virtual method from class which shouldn't be subclassed"

This reverts commit 858129d.

I remember now... that was added to avoid a totally nonsense and
otherwise unfixable crash in the sip bindings...

python/core/qgsproperty.sip

@@ -80,6 +80,8 @@ class QgsProperty
     };
 
     QgsProperty();
+    virtual ~QgsProperty();
+
     static QgsProperty fromExpression( const QString& expression, bool isActive = true );
     static QgsProperty fromField( const QString& fieldName, bool isActive = true );
     static QgsProperty fromValue( const QVariant& value, bool isActive = true );

src/core/qgsproperty.h

@@ -201,6 +201,8 @@ class CORE_EXPORT QgsProperty
      */
     QgsProperty();
 
+    virtual ~QgsProperty() = default;
+
     /**
      * Returns a new ExpressionBasedProperty created from the specified expression.
      */