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.
Where this should live is a good question. It's an extension method to `Player` (because `OpenRA.Game` can't depend on `OpenRA.Mods.Common`. The `Player` class is located directly in the `OpenRA` namespace (hence the `namespace OpenRA` above), so it could probably live in the root of `OpenRA.Mods.Common`. But there's only one other source file there... I chose `OpenRA.Mods.Common/Traits/Player` because it was specific to players. I'll go with what you throw at me.