[IZPACK-1120]

This is to address http://jira.codehaus.org/browse/IZPACK-1120

Added a displayHidden attribute for both <panel> tags and <field> tags for the userInputSpec.xml file.

1\. Setting displayHidden attribute in the <panel> makes all fields within the panel be shown regardless of what the field's conditionid evaluates to. But if the conditionid evaluates to false, display the field as disabled.
```
<panel id="a.panel" displayHidden="true">
<field type="check" variable="b" conditionid="b.cond">
    <spec id="b.id" true="true" false="false"/>
</field>
<field type="check" variable="c" conditionid="c.cond">
    <spec id="c.id" true="true" false="false"/>
</field>
<panel>
```
2\. Setting displayHidden attribute in the <field> that specific field within the panel be shown regardless of what the field's conditionid evaluates to. But if the conditionid evaluates to false, display the field as disabled.
```
<panel id="a.panel">
<field type="check" variable="b" conditionid="b.cond" displayHidden="true">
    <spec id="b.id" true="true" false="false"/>
</field>
<field type="check" variable="c" conditionid="c.cond">
    <spec id="c.id" true="true" false="false"/>
</field>
<panel>
```