rename icon to appid and use it in about dialogue
I am coming from https://github.com/flathub/flathub/pull/1298#discussion_r359819846 and the request is to rename the icons in line with the app-id, e.g. org.gnome.gThumb.
I am coming from https://github.com/flathub/flathub/pull/1298#discussion_r359819846 and the request is to rename the icons in line with the app-id, e.g. org.gnome.gThumb.