Blockquote's withText method

[JuhG]

The withText method in the Blockquote class doesn't do anything currently. The content saved to the text property is not used anywhere and the isValid method returns always false. This is because they are using the parent TextContainer's methods.

One way of solving this would be to make withText a simple wrapper around appendText, the other is to make a custom isValid method and modify the toDOMElement method to use the text property.