plug-ins: add a "palette" argument to palette-to-gradient.

Older palette-to-gradient used to have a palette argument. Bring it
back. The logics is that if the string is empty or None, the procedure
will use the currently selected palette. Otherwise it will try to
transform the named palette into a gradient and will fail with an error
if this palette does not exist.

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

@@ -49,8 +49,17 @@ def make_gradient(palette, num_segments, num_colors):
     return gradient
 
 def run(procedure, args, data):
-    palette = Gimp.context_get_palette()
-    (_, num_colors) = Gimp.palette_get_info(palette)
+    # Get the parameters
+    palette = None
+    if args.length() > 1:
+        palette = args.index(1)
+    if palette == '' or palette is None:
+        palette = Gimp.context_get_palette()
+    (exists, num_colors) = Gimp.palette_get_info(palette)
+    if not exists:
+        error = 'Unknown palette: {}'.format(palette)
+        return procedure.new_return_values(Gimp.PDBStatusType.CALLING_ERROR,
+                                           GLib.Error(error))
 
     if procedure.get_name() == 'python-fu-palette-to-gradient':
         num_segments = num_colors - 1
@@ -76,18 +85,30 @@ def run(procedure, args, data):
     return retval
 
 class PaletteToGradient (Gimp.PlugIn):
-    ## Properties: parameters ##
+    ## Parameter: run mode ##
     @GObject.Property(type=Gimp.RunMode,
                       default=Gimp.RunMode.NONINTERACTIVE,
                       nick="Run mode", blurb="The run mode")
     def run_mode(self):
-        """Read-write integer property."""
+        '''The run mode (unused)'''
         return self.runmode
 
     @run_mode.setter
     def run_mode(self, runmode):
         self.runmode = runmode
 
+    ## Parameter: palette ##
+    @GObject.Property(type=str,
+                      default=None,
+                      nick= _("Palette"))
+    def palette(self):
+        '''Palette name or empty string for the currently selected palette'''
+        return self.palette
+
+    @palette.setter
+    def palette(self, palette):
+        self.palette = palette
+
     ## Properties: return values ##
     @GObject.Property(type=str,
                       default="",
@@ -134,6 +155,7 @@ class PaletteToGradient (Gimp.PlugIn):
             # 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_menu_path ('<Palettes>')