Add framework for supporting geometrical occluders within rooms, and add support for sphere occluders.
Includes gizmos for editing.
They also work outside the portal system.
This PR makes the 'convert rooms' button permanently on the toolbar and accessible whichever node is selected, so you can convert rooms without having to select the RoomManager first.
It also adds a togglable item 'view portal culling' to the 'View' menu which is a simple way of setting the RoomManager 'active' setting without the RoomManager being the selected node.
Both of these have keyboard shortcuts, which should make it much faster to reconvert rooms and edit.
In addition there the string in the 'Perspective' Listbox is modified to show [portals active] when portal culling is operational, for visual feedback. This is updated when you change modes, and when the rooms are invalidated.
Allows users to have the RoomManager as the roomlist.
Fixes a couple of bugs dealing with situations where users attempt to link Portals to Rooms outside the roomlist.
Adds a PortalEditorPlugin allowing you to flip individual portals.
