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.
This has been intentionally left out from TD because (a) I think it is an anti-feature that hurts polish and gameplay, and (b) there isn't any ingame UI for it anyway.