PpPpdOptionWidget and PpIppOptionWidget both use combo boxes for certain types of selections. With GTK4, combo boxes no longer support scrolling (gtk#3674 (closed)), which in turn causes problems setting some things in the PpOptionsDialog (#1704 (closed)).
This replaces instances of GtkComboBox with GtkDropDown which do support scrolling. This change was applied to both PpIppOptionWidget and PpPpdOptionWidget as both are used in PpOptions dialog.
Since the configuration values passed to CUPS can no longer be stored in a GtkTreeModel alongside the displayed values, some logic changes to update_widget_real in PpPpdOptionWidget were made to maintain the reference to the ppd_option_t so the selected index can be mapped to the configuration value.
Fixes #1704 (closed)