Mezon Field: Base class for form fields in Mezon based HTML GUI

This is a base class for form fields in Mezon based HTML GUI.

It provides several functions that should be redefined in sub-classes to implement features of HTML form based user interfaces. Currently it provides functions to:

- Return the HTML to render the field
- Return the field title
- Return if it is a required field in a form
- Return if it is a visible field in a form
- Return if the field should occupy a whole form row
- Return the field type
- Return the field name prefix