Printer properties dialog doesn't handle custom values for pickOne options

Some printer PPDs define "pick one" (i.e. combobox) options where one option is "Custom".

When selecting a custom option a text input should appear to enter a custom value.

QPrintDialog doesn't show that text field, so it's not possible to enter custom values.

See this KDE bugreport: https://bugs.kde.org/show_bug.cgi?id=464101

An example PPD file that makes use of this can be found at https://www.openprinting.org/ppd-o-matic.php?driver=pxlmono-Ricoh&printer=Ricoh-Aficio_1022&show=1