Adds framework for multiple UI styles for humans, with a no-UI mode being the first example. Also makes all mobs' UIs scalable.

- Rewrote all UI element positions to use values that are anchored to NORTH, SOUTH, EAST, WEST and CENTER instead of absolute numbers. Changing view range now no longer messes up the UI.
Screenshots:
http://www.ss13.eu/uiscaling_human.png (human)
http://www.ss13.eu/uiscaling_borg.png (cyborg)
http://www.ss13.eu/uiscaling_alien.png (alien)
http://www.ss13.eu/uiscaling_monkey.png (monkey)
- Changing the view range no longer messes up shortsightedness and welding mask dither effects
Screenshots:
http://www.ss13.eu/uiscaling_welding.png (welding mask)
http://www.ss13.eu/uiscaling_shortsight.png (shortsightedness)
- The flash overlay now covers the entire screen no matter what the view range is
- The damage overlay and blindness overlay are now centered even if you change the view range. They are however images of a fixed size, so they only cover a part of the screen. This one is better than it was, but still 'imperfect'. I'm guessing we'll have to wait until we switch to byond 500 for a better solution to this.
Screenshot:
http://www.ss13.eu/uiscaling_dead.png (critical condition)
- Pressing F12 now no longer only switches between the standard and a reduced overlay, it cycles through all available UI styles, of which there are currently three: Standard UI, reduced UI and no UI (intended for taking screenshots while alive). Note that this does not remove the blindness, damage and flash overlays, it however removes all damage indication icons (like suffocation, fire damage, etc)
Screenshot:
http://www.ss13.eu/uiscaling_blank.png (no UI)