Replace Window with WindowProxy in IDL throughout?

[awesomekling]

The HTML spec returns WindowProxy for Document.defaultView: https://html.spec.whatwg.org/#dom-document-defaultview

Should the uses of Window in UIEvents IDL also be WindowProxy instead?

FWIW WebKit has been using WindowProxy since 2018: https://bugs.webkit.org/show_bug.cgi?id=185022

[jamierocks]

Related PR (albeit in the other direction): #379