Draft: export_dialog: Add a label stating the license of the icon

All the icons come from icon-development-kit (https://gitlab.gnome.org/Teams/Design/icon-development-kit/), which is licensed CC0.

State that, so it’s clear that there are no restrictions on reuse or attribution when using icons from Icon Library.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #31 (closed)