Add UIEvent::new_uninitialized().
Add MouseEvent::new_uninitialized().
auto merge of #2384 : saneyuki/servo/2383, r=Ms2ger

Fix #2383

These changes introduce `FooEvent::new_uninitialized()` constructor. This constructor use to create `FooEvent` without calling `FooEvent::InitFooEvent`.

#2383 says integrating `FooEvent::new()` and `InitFooEvent`, but we need to preserve simple `FooEvent::new_uninitialized()` for `document.createEvent()`.

@jdm r?