Some structures shouldn't be ConquestCritical

[elexis]

Currently you are defeated in conquest mode, if all your units and structures are destroyed, that have the ConquestCritical tag. Every unit and structure has this tag, except some that remove it explicitly:

other/obelisk
civic_house
defense_defense_tower
defense_outpost
defense_wall_gate
defense_wall_tower
defense_wall
economic_farmstead
economic_market
economic_storehouse
resource
unit_fauna
unit_support_trader

This list is incomplete. It should contain the following structures as well:

corral
blacksmith
rotary mill
pillar
theather (athen & sparta)
library (macedonian & seleucids)
lighthouse (ptolemian)

Docks and ships are another way to survive the defeat currently. But I think this is okay, since the player might have resources left and help his teammates on the sea. Also notice that the athenian tireme can produce units.

The wooden tower and palisade are alreay covered by the defense_defense_tower and defense_wall structure.

What about removing the trader of the list? I can imagine it would be fun if you could still trade between your allied markets with your leftover trade carts (similar to fishing with your leftover docks).

Alternatively one could define the Conquest mode in such a way that you will be defeated if and only you don't have a CC and can't rebuild it, even if you had the resources. In that case, docks and ships (except athenian tireme) would have to be added to the list as well.

Refs #565, #911, r9923 and r10357.