So that we don't have to depend on GNOME shell. https://bugzilla.gnome.org/show_bug.cgi?id=764001
Previously we were synthesising an end date of "start + 1 day", but this isn't necessary. RFC 2445 tells us that no end date already means an all day event. Check this case directly. https://bugzilla.gnome.org/show_bug.cgi?id=762985