Buildings: Support Linked Spawners/Units

[CWolfs]

Buildings have the concept of Linked Spawners. When a building is destroyed it will also destroy any units associated with a Linked Spawner.

• Add support to MC and the Designer to support this for MC/Designer Props.
• Consider how this support could be added to vanilla buildings and, if possible, support it

[CWolfs]

Since Props are EncounterLayer specific then the info should be stored in the EncounterLayer override json.

The Designer will need the ability to select vanilla Buildings to be able to get their GUID, and any other associated/important information.

The Designer will need the ability to add data to vanilla Buildings (currently only Prop Buildings can have data added to them).

The LinkedSpawners are UnitSpawnPointGameLogic (so individual unit spawn points).