2.99 ScriptFu: add custom widgets to GimpProcedureDialog

For version 3 scripts, using script-fu-register-filter

Widgets for SF-OPTION and SF-DIRNAME

Fixes #9992

Test case is Sphere v3.

Note that SF-VECTORS is still not supported,
until there is a VectorsChooser widget in libgimp.

A step towards deprecating old-style scripts using script-fu-register,
and deleting script-fu-interface, the duplicate dialog for SF.
But we still need to change or replace script-fu-register (for non-filter plugins)
to use GimpProcedureDialog.

plug-ins/script-fu/libscriptfu/script-fu-dialog.c

@@ -27,6 +27,7 @@
 #include "script-fu-command.h"
 
 #include "script-fu-dialog.h"
+#include "script-fu-widgets-custom.h"
 
 
 /* An informal class that shows a dialog for a script then runs the script.
@@ -159,13 +160,11 @@ script_fu_dialog_run (GimpProcedure        *procedure,
                                       script_fu_script_get_title (script));
   /* dialog has no widgets except standard buttons. */
 
-  /* It is possible to create custom widget where the provided widget is not adequate.
-   * Then gimp_procedure_dialog_fill_list will create the rest.
-   * For now, the provided widgets should be adequate.
-   */
+  /* Create custom widget where the stock widget is not adequate. */
+  script_fu_widgets_custom_add (dialog, script);
 
   /* NULL means create widgets for all properties of the procedure
-   * that we have not already created widgets for.
+   * that we have not already created custom widgets for.
    */
   gimp_procedure_dialog_fill_list (dialog, NULL);