Auto merge of #17578 - pyfisch:input-event, r=jdm

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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components/script/dom/mod.rs

@@ -372,6 +372,7 @@ pub mod htmlulistelement;
 pub mod htmlunknownelement;
 pub mod htmlvideoelement;
 pub mod imagedata;
+pub mod inputevent;
 pub mod keyboardevent;
 pub mod location;
 pub mod mediaerror;