keyboard: Honor immutable command/name in custom commands

If the gsettings key for the command or name of a custom shortcut is not
writable, the keyboard panel still lets you edit them. This commits
makes the corresponding widgets unsensitive when the gsettings key is
not writable.

https://bugzilla.gnome.org/show_bug.cgi?id=749381

panels/keyboard/keyboard-shortcuts.c

@@ -986,6 +986,10 @@ edit_custom_shortcut (CcKeyboardItem *item)
 
   g_settings_delay (settings);
 
+  gtk_widget_set_sensitive (custom_shortcut_name_entry,
+                            item->desc_editable);
+  gtk_widget_set_sensitive (custom_shortcut_command_entry,
+                            item->cmd_editable);
   gtk_window_present (GTK_WINDOW (custom_shortcut_dialog));
   result = gtk_dialog_run (GTK_DIALOG (custom_shortcut_dialog));
   switch (result)