Currently, we can not use Gimp.Choice as a parameter type for
Python plug-ins. This means that we're using a string instead for
the "output-format" parameter, which creates a textbox in the GUI.
This patch manually creates a GimpStringComboBox in the GUI and
syncs it with the "output-format" string parameter. Once Gimp.Choice
can be used in Python, we should be able to swap this out without
impacting plug-in developers (since it would also take in a string)
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