The default filter, that is, the filter that is initially active in the file chooser dialog.
If the default filter is null, the first filter of gtk.file_dialog.FileDialog.filters is used as the default filter. If that property contains no filter, the dialog will be unfiltered.
If gtk.file_dialog.FileDialog.filters is not null, the default filter should be part of the list. If it is not, the dialog may choose to not make it available.
Builder instance for fluent chaining
Set defaultFilter property.