FGU extension for CoreRPG (any ruleset built on it).

Fixes two stock CoreRPG Location deficiencies: dragging a location onto another copies it instead of linking it, and sharing a location with players is all-or-nothing (no way to reveal a parent without exposing every child, or vice versa).

• Any CoreRPG-based ruleset
• Purely additive — doesn't remove or replace any stock Location functionality