Add EventLoopBuilder

This commit adds an `EventLoopBuilder` struct to simplify event loop
customization and providing options to it upon creation. It also
deprecates the use of `EventLoop::with_user_event` in favor of the same
method on new builder, and replaces old platforms specific extension
traits with the new ones on the `EventLoopBuilder`.

src/window.rs

@@ -83,7 +83,7 @@ impl WindowId {
     }
 }
 
-/// Object that allows you to build windows.
+/// Object that allows building windows.
 #[derive(Clone, Default)]
 pub struct WindowBuilder {
     /// The attributes to use to create the window.