Allow a displayHidden attribute within the <panel> tags of
UserInputPanel.
- When displayHidden is set to true all fields in this panel will be
  shown regardless if their conditionid is true or false
- If a field's conditionid is false but displayHidden is set to true,
  show the field but disable it
Allow to use displayHidden attribute on individual fields aswell as for entire panels
Give ability to allow disabling any FileInputField object.
Updaing xsd for displayHidden attribute
Ensure TestFieldConfig adheres to updated FieldConfig interface.
Merge branch 'master' into IZPACK-1120

Conflicts:
	izpack-dist/src/main/resources/schema/5.0/izpack-userinput-5.0.xsd
	izpack-panel/src/main/java/com/izforge/izpack/panels/userinput/field/Field.java
Merge pull request #219 from mtjandra/IZPACK-1120

[IZPACK-1120]
Merge branch 'master' of https://github.com/izpack/izpack into IZPACK-1085