Added persistency for the UserInputPanels.
- When hitting the previous button we attempt to save contents of the
  panel into the install data if the saveData() method has been
  implemented for the panel.
- There was inconsistency where some fields like text/radio would be
  saved on previous button, but not password fields.
- This can be useful when a user has to switch back and forth between
  panels and does not want to loose their data.
Added persistency to TargetPanel
Ensure file and directory fields are not expanded when saving without validation. Less confusion for persistency
Properly persist password fields, and no longer remake the userinputpanel everytime it is shown.
Adding new attribute 'updateTrigger' for dynamic variables
- updateTrigger="variable" where variable refers to a variable within idata.
- Indicates that variable should be re-evaluated only when the specified
  update trigger has changed.
Revert "Adding new attribute 'updateTrigger' for dynamic variables"

This reverts commit bc24ef5e895c493da8f0ab3008c410981cb9dc39.
BuildUI must run on panelActivate, otherwise fields may still remain
hidden even if their conditionid is true. The bug can be reproduced when
the condition is set to true from a previous panel rather than on the
same panel.
Merge pull request #210 from mtjandra/IZPACK-1112

[IZPACK-1112]