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/sync_object.rs

@@ -6,3 +6,8 @@ fn window_sync() {
     // ensures that `winit::Window` implements `Sync`
     needs_sync::<winit::window::Window>();
 }
+
+#[test]
+fn window_builder_sync() {
+    needs_sync::<winit::window::WindowBuilder>();
+}