Add polish to Intervention

[JovialFeline]

I split a lot of the beachhead-triggered stuff into its own function, so the diff looks messier than it should be. Please pardon that.

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• Added a persistent death check for the MCV so its loss can trigger defeat if the beachhead is not yet reached. This includes its sale as a Construction Yard or its death as an LST passenger.

  • Fixes Intervention: losing MCV loaded in transport doesn't fail mission #8294

• Added a basic Easy difficulty. On Easy, the Air Force HQ is revealed (but left in fog), and a flare marks the beachhead after a Naval Yard is built. This flare is coupled with a speech notification.

• Added speech notifications for completing/failing the beachhead objective.

• Changed the player's starting cash to 2400 (from 2000). This should trim 20-30 seconds from the slow start, because 2 ore hauls are needed for the Naval Yard and LST instead of 3.

• Added some bad luck insurance to the village objective. In the unlikely event that the last house is destroyed after the Air Force HQ is captured, the player is still granted victory.

• Changed Soviet aircraft to linger for less time before flying away for disposal (25 ticks from 150).

• Swapped a Map.ActorsInWorld check for an Allies.Actors() check because that seems more sane.

[obrakmann]

These changes work well. In hindsight, the slow start was probably a stupid idea. I would not mind getting rid of it and instead just have a transport waiting on the beach.