Add support for exporting to opml format
Adds support for exporting a list of podcasts
in opml format. In order to make this functional in
flatpak it uses a FileChooserDialog over FileChooserNative,
and the --filesystem=home permission. FileChooserNative
has issues saving files: https://github.com/flatpak/xdg-desktop-portal/issues/230
Related to #41 (closed)
Edited by Christopher Davis