Short Game option

Adds a short game lobby and map option. 

The short game is basically what we already had, players need only destroy an enemy's base buildings to win. In a "long" game, they need to destroy (almost) all enemy units to win. The exception are cloaked units and buildings (that is, those that are cloaked by default, not upgraded ones), which are not taken into account.

Also fixes #6589.
It kind of does. In this example, the map ends when you kill the apc a player starts off with (even before this change). If that were changed to a long game you'd have to kill everything your opponent has picked up.  I'm not sure what was intended, or what is better, so I'll just go with the suggestions you throw at me.