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/platform_impl/windows/mod.rs

@@ -2,8 +2,10 @@
 
 use winapi::{self, shared::windef::HMENU, shared::windef::HWND};
 
-pub use self::{
-    event_loop::{EventLoop, EventLoopProxy, EventLoopWindowTarget},
+pub(crate) use self::{
+    event_loop::{
+        EventLoop, EventLoopProxy, EventLoopWindowTarget, PlatformSpecificEventLoopAttributes,
+    },
     icon::WinIcon,
     monitor::{MonitorHandle, VideoMode},
     window::Window,