notify: Use GBytesIcon
Previously we sent a GFileIcon for notifications, but the flatpak portal does not support GFileIcon (https://github.com/flatpak/xdg-desktop-portal/issues/317). Because of this notifications from the flatpak were being dropped.
Now we create a GBytesIcon by loading a GFile and reading the bytes, and set that as the icon for notifications.
Closes #490 (closed)