filechoosernative: Make get_filter work for (non-portal) native chooser

filechoosernative: Make get_filter work for (non-portal) native chooser

'gtk_file_chooser_get_filter' did not work for GtkFileChooserNative,
since the previous way did not properly handle the delegate dialog,
s.a. commit a136cbae8f8848d49976512f7f0ec73137428ea0
("filechoosernative: forward current_filter to delegate dialog",
2018-11-29) for details, wich basiscally fixed the same thing for
the 'gtk_file_chooser_set_filter' case.

This fixes #1820 for the fallback dialog. A solution for the portal
one (which also requires changes to xdg-desktop-portal and
xdg-desktop-portal-gtk as well) will be suggested in a subsequent step.

Bug: #1820