Make WindowBuilder Send + Sync

Window builder is always accessed by winit on the thread event loop
is on, thus it's safe to mark the data it gets as `Send + Sync`.
Each unsafe object is marked individually as `Send + Sync` instead
of just implementing `Send` and `Sync` for the whole builder.

tests/send_objects.rs

@@ -16,6 +16,11 @@ fn window_send() {
     needs_send::<winit::window::Window>();
 }
 
+#[test]
+fn window_builder_send() {
+    needs_send::<winit::window::WindowBuilder>();
+}
+
 #[test]
 fn ids_send() {
     // ensures that the various `..Id` types implement `Send`