Implement the InputEvent
Add WebIDL, from UI Events working draft.
Implement script type for event.
Note that actually firing InputEvents is not part of this commit.
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As part of my effort to improve [keyboard handling](https://groups.google.com/forum/#!topic/mozilla.dev.servo/e6R7eBDjo4M) it is necessary to introduce the InputEvent. While I have a working prototype using the input event this still needs a lot of work before it can be merged into servo.
cc @jdm
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