Arcades, overlays and Git being evil

Added the most important damn checks of this entire PR that got eaten by
Git when I synced
Added a visual indication of a pAI being paired with a computer - a tiny
blue light at the bottom
Forbid to pair with computers that have non-standard icons, except the
arcade machine.
When the arcade is emagged, if the pAI loses, it will explode like a
detonated PDA. If it's slotted into a PDA, it will explode with it.
Severs connection if the pAI and the computer are on different z-levels