[IZPACK-1069] Allow dynamic views on panels

This is to address http://jira.codehaus.org/browse/IZPACK-1069
Refer to the link above for more information.

Allows a checkbox or radio button to show additional fields on the same userInputPanel, if conditions specified correctly. Ensures that fields are not validating during this process, validation only needs to occur when pressing the next button.

Allow specifying the topBuffer as number of pixels with the "rigid" attribute. This comes naturally and dynamically showing and hiding fields make the spacing between the top field and title change.

Let me know if anything needs clarification, and am fully open to feedback!