[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