Wheelchairs

### Wheelchairs
Adds wheelchairs as introduced on the forum [here](http://baystation12.net/forums/viewtopic.php?f=5&t=10521).
Some more details:
 * Crew spawning missing both feet will spawn with a wheelchair.
 * Replaces stool in Medical Examination with a wheelchair.
 * Moving with a wheelchair will ignore damage to legs and feet.
 * Movement with a wheelchair and best health conditions will be slighty faster than normal character movement with a missing foot.
 * Clicking and dragging the wheelchair sprite will make the user actively take control of the wheelchair
 * Bumping into something solid while the driver is on harm intent will stun the occupant for about a second and throw them into the obstacle.
 * Wheel- and Officechairs: Bumping into an obstacle when the chair has been propelled with a fire extinguisher will stun the occupant for a few second and throw them into the obstacle. If the obstacle was a living mob, they will be stunned as well and experience slight brute damage.

A note on the balance:
People spawning in wheelchairs (and thus having no feet) will not be able to buckle in again once they were unbuckled in any way. I left this as it is with the idea that spawning as a feetless character should have some more downsides other than the slight movement restriction, making them more dependent on other crewmembers in some way.
I might also add a check for space tiles in near future so nobody gets the idea to go EVA with a wheelchair.