Engine: added support to portals (invisible interactive props with dynamic sizing and naming)
Engine: added Portal utility function Portal.checkIfIs("portalName")
Inventory: added utility function Inventory.isMouseOver()
Engine: added support to mouse cursor overriding by Prop subclasses; added Door cursor
Engine: changed protection level of internal Prop's _onInteract or _onItemUse to protected, to allow sub-class overriding
Engine: Portals can now be set to automatically transition to a certain scene on interact
Engine: added Portal utility function Portal.placeOnScene()
StartingScene: added portals to P1 and P2, as well as footsteps