Replaced `close` on Window trait with `Set<ShouldClose>`
Replaced `close` on Window trait with `Set<ShouldClose>`

Closes https://github.com/PistonDevelopers/event/issues/269

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