Implement #1290 "Ability to specify fr-language on PDF URL"

- propagate "fr-language" when processing PDF
- add "parameters" parameter to "send" action
- add xxf:append-query-params()
- add "fr-remember-language" URL parameter
- remove deprecated "orbeon-portlet" URL parameter
- move navigation logic / mode changes in navigation-model.xml to Scala processes
- move instant PDF logic to pdf-model.xml
- some refactoring