app: some cleanup/refactoring in gimp_param_spec_duplicate()
app, libgimp*: pass translated blurbs to GIMP_CONFIG_INSTALL_PROP_FOO()

instead of passing N_()-strings; and remove gettext() calls on these
strings when using them. Reduces complexitx, and fixes double- and
untranslated strings. Also enables to treat properties of GIMP and
GEGL objects the same way, which was totally broken before.