[gimp] Corrected code to work with newer split Gimp and GimpUi bindings.

[Jehan <jehanp src gnome org>]

commit c16e310c368fd3bb9b4bf67c82e4b50bd6128830
Author: PhotonQyv <shenstone porter gmail com>
Date:   Fri Jun 5 15:12:34 2020 +0000

    Corrected code to work with newer split Gimp and GimpUi bindings.

 plug-ins/python/palette-offset.py | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)
---
diff --git a/plug-ins/python/palette-offset.py b/plug-ins/python/palette-offset.py
index 14d8c6b074..aa169a194a 100644
--- a/plug-ins/python/palette-offset.py
+++ b/plug-ins/python/palette-offset.py
@@ -16,6 +16,8 @@
 import gi
 gi.require_version('Gimp', '3.0')
 from gi.repository import Gimp
+gi.require_version('GimpUi', '3.0')
+from gi.repository import GimpUi
 from gi.repository import GObject
 from gi.repository import GLib
 from gi.repository import Gio
@@ -132,10 +134,10 @@ class PaletteOffset (Gimp.PlugIn):
             gi.require_version('Gtk', '3.0')
             from gi.repository import Gtk
 
-            Gimp.ui_init ("palette-offset.py")
+            GimpUi.ui_init ("palette-offset.py")
 
             use_header_bar = Gtk.Settings.get_default().get_property("gtk-dialogs-use-header")
-            dialog = Gimp.Dialog(use_header_bar=use_header_bar,
+            dialog = GimpUi.Dialog(use_header_bar=use_header_bar,
                                  title=_("Offset Palette..."))
 
             dialog.add_button("_Cancel", Gtk.ResponseType.CANCEL)
@@ -151,7 +153,7 @@ class PaletteOffset (Gimp.PlugIn):
             label.show()
 
             amount = self.set_property("amount", amount)
-            spin = Gimp.prop_spin_button_new(self, "amount", 1.0, 5.0, 0)
+            spin = GimpUi.prop_spin_button_new(self, "amount", 1.0, 5.0, 0)
             spin.set_activates_default(True)
             box.pack_end(spin, False, False, 1)
             spin.show()