po-script-fu: update all script-fu's po with recent xgettext.

Current dev version of gettext has a fix for GIMP's script-fu syntax.
See Bug 720052.
Bug 720052 - Make GIMP recommend a recent gettext.

Script-fu gettext strings were mostly not extracted by xgettext. This
will be fixed in upcoming gettext 0.19. Add a test to warn packagers
using older gettext.
(cherry picked from commit feb1bf2797d87a24bea1ddc68eafac2db2e83ce4)