Adding functionality to FormTextFieldDelegate

This add 2 things:

1. More acces to the textField properties

   • readOnly => usefull to show the user that this text can be modified but control the way it's done. Example on Klevernotes settings page (which currently use a MouseArea as a workaround).
   • fieldFocus (alias textField.focus) => this enable, for example, the statusMessage to be hidden once the user gives focus to the textField. Example on Krone transaction page (property name is a bit different).

2. maxLength property. When maxLength > 0 => the length of the text inside the textField can't be greater than maxLength