filechoosernative: Implement gtk_file_chooser_set_filter()
gtk_file_chooser_set_filter() doesn't work for GtkFileChooserNative
The code forwards added and removed filters to the delegate dialog, but doesn't do anything to set the selected one, so the wrong one gets chosen. So let's do that.
This merge request includes two separate fixes: one to fix the fallback dialog, and one to fix the portal dialog. The fix for the portal dialog depends on these two pull requests:
Fixes #1492 (closed)