Don't use gtk_file_chooser_set_filename() for save file dialogs.

This seems to be broken in old GTK+ versions, so use
gtk_file_chooser_set_current_name() and gtk_file_chooser_set_current_folder()
for save file dialogs which seem to work in all versions.

Closes #15133.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73932 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775