Helper class that manages i18n files for gadgets.
2 modes are supported :
Static mode :
gadget has to statically include the Local headers and associated files
Dynamic mode : gadget only includes a dynamic_messages.properties This file
is used to know what labels are used by the gadget (and provide default
value) The Headers in Gadgets specs and associated XML files will be
dynamically generated using the message bundles used by Nuxeo app.