Cleaner display for 'trig'.

This add two kinds of triggers:
 $page->trigError()
 $page->trigSuccess()

The old $page->trig() takes 2 arguments:
 - the message to display
 - the kind of message ('errors' or 'success'), default is errors in order
   to preserve backward compatibility

Plan is to remove all ->trig() calls and switch the trig() method to protected
access rights.

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>