Feature: Compo flavor "Demoshow"

[chainq]

Most demoparties I organize contain demoshows, and it would be great to be able to utilize Wuhu's compo-slide-system during the demoshow. However, this is difficult, because adding the demoshow as a compo has several implications:

• the demoshow added as a "compo" shows up as a compo for uploading, voting, etc. maybe only on the admin interface, but still very distracting during the rest of the party. it can also easily leak with a misclick.
• it shows up as a "compo" in the timetable, while it isn't.
• the slide system will call it a "compo" due to several hardwired strings (if I remember correctly) at various places.

Additional consideration: probably should/could be implemented as a plugin? If a party doesn't need it, don't add it.