Unify metadata

[zoeyfyi]

As mentioned in #191

• https://forum.snapcraft.io/t/using-external-metadata/4642
• https://snapcraft.io/docs/desktop-menu-icon-support

[zoeyfyi]

The snap file now takes some metadata from the appstream file, but still requires a separate desktop file since I couldn't get it to do the renaming "magic" on the icon path.