Draft: custom print options page when printing through the portal

This will fix #1015, concerning the lack of support for our custom options page in the print dialogue when printing through the portal.

Fixing this requires work to be done in the portal itself.

This wip/chpe/export-print-portal-custom-options branch is on top of !393 (merged) and implements this.

It depends on a lots of not-yet-upstream branches: you need

• xdg-desktop-portal/wip/print and
• either
  • xdg-desktop-portal-gtk/wip/print and
  • gtk/print-custom-options-gtk3 for gtk3,
• or
  • xdg-desktop-portal-gnome/wip/print and
  • gtk/print-custom-options-gtk4 for gtk4.

There is also a libportal/print branch to help test the portal changes.