Unlocks platform-specific attributes

[tomaka]

Motivations:

• Adds ability to use icons on windows #688
• Add the possibility to set the parent window ID on Linux or to say whether we prefer X11 or Wayland.

How to use it:

#[cfg(windows)]
use glutin::os::windows::WindowBuilderExt;

let mut window_builder = glutin::WindowBuilder::new();
window_builder.set_icon_path("icon.ico");   // available through the `WindowBuilderExt` trait
let window = window_builder.build().unwrap();