Reduce usage of pointers to VLC entities

Final goal (of multiple PR's) is to remove all remaining pointers from
serializeable game state, and replace them with either identifiers or
with shared/unique pointers.

CGTownInstance::town and CGHeroInstance::type members have been removed.
Now this data is computed dynamically using subID member.

VLC entity of a town can now be accessed via following methods:
- getFactionID() returns ID of a faction
- getFaction() returns pointer to a faction
- getTown() returns pointer to a town

VLC entity of a hero can now be accessed via following methods:
- getHeroTypeID() returns ID of a hero
- getHeroClassID() returns ID of a hero class
- getHeroType() returns pointer to a hero
- getHeroClass() returns pointer to a hero class

lib/serializer/ESerializationVersion.h

@@ -63,6 +63,7 @@ enum class ESerializationVersion : int32_t
 	REGION_LABEL, // 864 - labels for campaign regions
 	SPELL_RESEARCH, // 865 - spell research
 	LOCAL_PLAYER_STATE_DATA, // 866 - player state contains arbitrary client-side data
+	REMOVE_TOWN_PTR, // 867 - removed pointer to CTown from CGTownInstance
 
-	CURRENT = LOCAL_PLAYER_STATE_DATA
+	CURRENT = REMOVE_TOWN_PTR
 };