Multipage forms class

Extension for the forms generation and validation class to manage forms
that are presented in multiple pages. It features:

- Sequential mode (Wizard mode): input fields appear in pages that are
accessed in sequence using navigation buttons "Continue >>", " "Finish >>" and "Cancel".
- Random access mode (Tabbed pages mode): input fields appear in pages
that may be accessed in any order using buttons at the top to go directly
to each page and buttons at the bottom "Modify", "Save" and "Cancel".
- Redefinable layout
- Redefinable navigation button and error message text strings.