lmno: Generalize the support for multiple game engines

lmno: Generalize the support for multiple game engines

Rather than having repeated lists of the engines, we have one list at
the top of the file and then use that list to iterate over all engines
to mount their sub-apps at a path that is named by engine.name.

We also fix the construction of the Game class to use an imported
constructor (so that each game engine has objects of its own class
instead of only having games constructed via empires.Game()
exclusively).