changes:
Light tubes now make a noise when turning on.
If a light loses power, it will switch to "emergency mode": the light will glow a dim red, drawing power from a small integrated cell that should last approximately 10 minutes. If the light has power, it will charge its cell.
Light flickers now play the flicker sound for each blink instead of once.
Split lighting.dm into three files: fixtures.dm, construction.dm, and bulbs.dm.
Light fixtures now use stat to represent on-state (as they can be off for multiple reasons): the on var has thus been removed.
Lightswitches now glow in the dark and make sounds when toggled.
Some code/idea for emergency lighting from TG, light sound from Interbay, lightswitch sounds from Bay.
Depends on #3948.
