[IZPACK-1112]

This is to address http://jira.codehaus.org/browse/IZPACK-1112
1. Save contents of fields when hitting the previous button
    - Even though we save the fields that may have 'invalid' values this is okay since we are not moving forward
    - The "saveData()" method should be used for a panel who wants to save values into installData when hitting previous
2. File and directory fields should not be expanded when hitting the previous button.
3. ~~New 'updateTrigger' attribute added for variables contained between the <dynamicvariables> tags~~
    - ~~This attribute indicates that the varaible should only be updated if the variables defined from 'updateTrigger' has been updated.~~
    - ~~Use case if you have "${INSTALL_PATH}/this/location" as your value and you want to default to this location if later on the user goes back to the target panel to choose a different install path. But you don't want to loose any changes the user has made to this variable if they have not changed their install path~~