Add interface HasMaxLength

[rmuller]

Describe your motivation

Some input components (EmailField, PasswordField, TextArea, TextField) have the property "maxLength". However, there is no inteface for this behavior. This is convenient when you create components based on meta data. Now you are forced to check if it is an instance of one of these types.

Describe the solution you'd like

Create an interface in line with the other interfaces available in Vaadin.

interface HasMaxLength {

    int getMaxLength();

    void setMaxLength(int maxLength);

}