Fix for IZPACK-487 - Processor available for all field types

Calling process() from Field.setValue() enables support for Processor for all Field types.

eg:

        <field type="dir" variable="selectedFolder">
            <spec create="true" mustExist="false" set="C:/default/folder" size="25" txt="Select a folder">
                <processor class="my.project.izpack.PathProcessor"/>
            </spec>
        </field>

Custom class to replace back-slashes with forward-slashes (a &lt;jar> element needs to be added to install.xml):

    public class PathProcessor implements Processor {
        @Override
        public String process(ProcessingClient client) {
            return client.getText().replaceAll("\\\\", "/");
        }
    }