python: Port arguments code to use functions

This patch ports the first-party Python plug-ins
to use the new argument API.
This restores the ability to add dropdown menus from
enums and Gimp.Choice parameters, and also allows
defaults to be set for custom datatypes like Gegl.Color.

plug-ins/python/palette-to-gradient.py

@@ -111,43 +111,6 @@ def run(procedure, config, data):
     return retval
 
 class PaletteToGradient (Gimp.PlugIn):
-    ## Parameter: run mode ##
-    @GObject.Property(type=Gimp.RunMode,
-                      default=Gimp.RunMode.NONINTERACTIVE,
-                      nick="Run mode", blurb="The run mode")
-    def run_mode(self):
-        '''The run mode (unused)'''
-        return self.runmode
-
-    @run_mode.setter
-    def run_mode(self, runmode):
-        self.runmode = runmode
-
-    ## Parameter: palette ##
-    @GObject.Property(type=Gimp.Palette,
-                      default=None,
-                      nick= _("_Palette"))
-    def palette(self):
-        '''Palette or None for the currently selected palette'''
-        return self.palette
-
-    @palette.setter
-    def palette(self, palette):
-        self.palette = palette
-
-    ## Properties: return values ##
-    @GObject.Property(type=Gimp.Gradient,
-                      default="",
-                      nick=_("The newly created gradient"),
-                      blurb=_("The newly created gradient"))
-    def new_gradient(self):
-        """Read-write integer property."""
-        return self.new_gradient
-
-    @new_gradient.setter
-    def new_gradient(self, new_gradient):
-        self.new_gradient = new_gradient
-
     ## GimpPlugIn virtual methods ##
     def do_set_i18n(self, procname):
         return True, 'gimp30-python', None
@@ -176,12 +139,17 @@ class PaletteToGradient (Gimp.PlugIn):
         if procedure is not None:
             procedure.set_attribution("Carol Spears, reproduced from previous work by Adrian Likins and Jeff Trefftz",
                                       "Carol Spears", "2006")
-            # We don't build a GParamSpec ourselves because passing it
-            # around is apparently broken in Python. Hence this trick.
-            # See pygobject#227
-            procedure.add_argument_from_property(self, "run-mode")
-            procedure.add_argument_from_property(self, "palette")
-            procedure.add_return_value_from_property(self, "new-gradient")
+
+            procedure.add_enum_argument ("run-mode", _("Run mode"),
+                                         _("The run mode"), Gimp.RunMode,
+                                         Gimp.RunMode.NONINTERACTIVE,
+                                         GObject.ParamFlags.READWRITE)
+            procedure.add_palette_argument ("palette", _("_Palette"),
+                                            _("Palette"),
+                                            GObject.ParamFlags.READWRITE)
+            procedure.add_gradient_return_value ("new-gradient", _("The newly created gradient"),
+                                                 _("The newly created gradient"),
+                                                 GObject.ParamFlags.READWRITE)
 
             procedure.add_menu_path ('<Palettes>/Palettes Menu')