[IZPACK-1096] Allow panel validators to be conditional

Add conditional validation to izpack panels.
Also allow to have more than one panel validators. Currently there can be more than one defined, but during compilation just one of them "wins" and the other validators are silently ignored by the compiler parser, which is bad.

Example:
```xml
<condition type="variable" id="Install">
    <name>installAction</name>
    <value>install</value>
</condition>
<condition type="variable" id="Update">
    <name>installAction</name>
    <value>update</value>
</condition>
<condition type="variable" id="Uninstall">
    <name>installAction</name>
    <value>remove</value>
</condition>

<panel classname="UserInputPanel" id="panel.example">
    <validator classname="com.mycompany.MyValidator1" condition="Install" />
    <validator classname="com.mycompany.MyValidator2" condition="Update" />
    <validator classname="com.mycompany.MyValidator3" condition="Uninstall" />
</panel>
```