Bug#775483: hexen: verify that the demo works in some packaged engine, and re-enable

[Alexandre Detiste]

> The next release of Chocolate Doom, ne Hexen, will have support for the
> 4-Level Demo Version. \o/

Cool

>> I'd be happy to swap it back to chocolate-doom (>= x.y) | hexen-engine

Shouldn't heretic-engine & hexen-engine be added to the official list
of virtual packages too ?
https://www.debian.org/doc/packaging-manuals/virtual-package-names-list.txt