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.

<!-- Please describe your changes on the following line: -->
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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tests/wpt/metadata/uievents/constructors/inputevent-constructor.html.ini

@@ -1,14 +0,0 @@
-[inputevent-constructor.html]
-  type: testharness
-  [InputEvent constructor without InputEventInit.]
-    expected: FAIL
-
-  [InputEvent construtor with InputEventInit where data is null]
-    expected: FAIL
-
-  [InputEvent construtor with InputEventInit where data is empty string]
-    expected: FAIL
-
-  [InputEvent construtor with InputEventInit where data is non empty string]
-    expected: FAIL
-

tests/wpt/mozilla/meta/MANIFEST.json

@@ -26519,7 +26519,7 @@
    "testharness"
   ],
   "mozilla/interfaces.html": [
-   "49dd9f6ef449813f2ce943d4c9fac351357e5c74",
+   "ffdc606aaf989ef8bcdecef8cef8764bbb2ae1b2",
    "testharness"
   ],
   "mozilla/interfaces.js": [

tests/wpt/mozilla/tests/mozilla/interfaces.html

@@ -140,6 +140,7 @@ test_interfaces([
   "HTMLVideoElement",
   "ImageData",
   "Image",
+  "InputEvent",
   "KeyboardEvent",
   "Location",
   "MediaError",