Add `flush_messages` function and remove `EventReceiver` type
Add `flush_messages` function and remove `EventReceiver` type

This simplifies the API and makes message flushing more generic.

PistonDevelopers/glfw-rs,bjz/glfw-rs,nstoddard/glfw-rs,mneumann/glfw-rs,derekdreery/glfw-rs,derekdreery/glfw-rs,servo/glfw-rs,bvssvni/glfw-rs,tomaka/glfw-rs