icons: update app icon
- fine tune metaphor
- more pleasing execution
- provides a 'Nightly' variant of the app icon (See GNOME/Initiatives#12 (closed))
Fixes https://gitlab.gnome.org/hughsie/gnome-firmware-updater/issues/13
Fixes https://gitlab.gnome.org/hughsie/gnome-firmware-updater/issues/13