Clicking on the Save button threw an exception as the wrong class was
used to invoke the `set_alternative_button_order_from_array()` method.
Since `Gtk.Dialog.set_alternative_button_order_from_array()` is
deprecated and the GIMP functions with the same name suppress
deprecation warnings in the C code, we suppress the warning here as
well. The `warnings.catch_warnings()` function is not thread-safe
according to the Python documentation, which should not pose a problem
here.
Also, the file chooser dialog now allows specifying a new file instead
of overwriting an existing file.
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