WEB: Rename indy4 to atlantis, to match the game ID used by ScummVM
WEB: Rename monkey1 to monkey, to match the game ID used by ScummVM
WEB: Link to the screenshots of Longbow

Also, add a category icon for Longbow
WEB: Add a blank category icon (mostly for unsupported games)
WEB: Add category icons to the demos page and fix some demo targets

Add a new optional tag, "category". If it's missing, the target name is
used instead
WEB: Add a download link to the Neverhood demo