Add waiting for events to core

[bvssvni]

See discussion #1109

This could be very simple, such as adding Window::wait_event https://github.com/PistonDevelopers/glutin_window/blob/master/src/lib.rs#L125

[bvssvni]

This could also be added to the default event loop as a setting.

[christolliday]

I looked into adding this to the various backends after adding it to just glutin_window, and I think the API I added there can be improved after looking at the other backends.

I previously added this method to GlutinWindow

fn wait_event(&mut self) -> Option<Self::Event>

but looking at the other backends, sdl2 and glfw, I think the best API is to have:

fn wait_event(&mut self) -> Self::Event
/// wait for an event, or for timeout to be reached
fn wait_event_timeout(&mut self, timeout: Duration) -> Option<Self::Event>`

The reasons are

• wait_event can wait for as long as it takes to get an event, so there is need to return an option.
• wait_event_timeout is especially useful for implementing a waiting loop that can still update at a fixed rate.
• of glutin, glfw, and sdl2, only glutin has a straightforward way I could see to implement wait_event_timeout without a dedicated method, and that's by waking it up from another thread via a WindowProxy. Both SDL2 and glfw provides an equivalent method to wait_event_timeout so that should be exposed.

[christolliday]

the glfw backend requires an extra method to be exposed via glfw-rs, so I made a pull request there PistonDevelopers/glfw-rs#358

If that and the piston changes get merged I can implement the new methods for all the various backends.

[bvssvni]

Closed by #1123