Passing `&RefCell<W>` instead of `&mut T`
Passing `&RefCell<W>` instead of `&mut T`

Closes https://github.com/PistonDevelopers/event/issues/240

PistonDevelopers/piston,getynge/piston,tempbottle/piston,placrosse/piston,caldwell/piston,bvssvni/event,bvssvni/piston,Potpourri/piston,axelmagn/piston,oldmanmike/piston,mitchmindtree/piston,bvssvni/piston,PistonDevelopers/piston,KevinFrans3/piston