LibWeb: Generate Window{Constructor,Prototype} from IDL

The Window object is massive, so let's do the conversion to IDL step
by step. First up: getting rid of the manual constructor and prototype
definitions, which can be generated from an empty `interface Window`.

Userland/Libraries/LibWeb/HTML/Window.idl

@@ -0,0 +1,6 @@
+#import <DOM/EventTarget.idl>
+
+// https://html.spec.whatwg.org/multipage/nav-history-apis.html#window
+[Global=Window, Exposed=Window, LegacyUnenumerableNamedProperties]
+interface Window : EventTarget {
+};