1. exitDim is now a WorldServer reference.

2. Added teleportation and remount queues
3. Added teleport code for ordinary and player entities.
1. TileGateway:

 - added validity check on each TE addition - this method is called the least often
 - Removed canUpdate override - it's checked before full TE init
 - added teleportation code which should handle player and non-player entities properly - though it doesn't work as expected
2. Cleaned Utils of dead code and moved entity transposition code to TileGateway
3. Added null check against hetHeldItem in player interact hook and stone platform creation around gateway in Nether

Issues:
- entity being teleported isn't stored to NBT properly for some unknown reason
- player gets glitch during teleport - not synced properly with destination world, hangs in void and re-emerges where he should be only after re-login