python: Minor enhancements to python plug-ins

* colorxhtml.py was converted to use
GimpProcedureDialog. A new aux
argument was added so that a widget
could be created in the GUI. The text
field and the file widgets were made
to be sensitive to the source-file bool to
better indicate to the user which one will
be used.

* palette-sort.py now uses babl instead of
colormath libraries to get CIE LAB and
CIE LCH(ab) values. This means all users
will be able to sort by those options even
if the colormath library isn't installed.

* histogram-export.py's out-format
widget was converted to a radio frame
as it is in 2.10.

plug-ins/python/histogram-export.py

@@ -169,7 +169,11 @@ def run(procedure, run_mode, image, n_layers, layers, config, data):
         GimpUi.init("histogram-export.py")
 
         dialog = GimpUi.ProcedureDialog.new(procedure, config, _("Histogram Export..."))
-        dialog.fill()
+
+        dialog.get_file_chooser ("file", Gtk.FileChooserAction.SAVE)
+        radio_frame = dialog.get_widget ("output-format", GimpUi.IntRadioFrame)
+
+        dialog.fill(None)
 
         if not dialog.run():
             return procedure.new_return_values(Gimp.PDBStatusType.CANCEL,