Add a feature to allow the previous button to be disabled for particular panels

In the example below the 'previous button' will be hidden in the shortcut panel

    <panels>
        <panel classname="CheckedHelloPanel"  id="hellopanel"/>
        <panel classname="HTMLLicencePanel"   id="licensepanel"/>
        <panel classname="ShortcutPanel"      id="shortcutpanel"      prevbutton="false"/>
        <panel classname="SimpleFinishPanel"  id="simplefinishpanel"/>
    </panels>