LibWeb+LibJS: Make the EventTarget hierarchy (incl. DOM) GC-allocated

This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.

There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.

Userland/Libraries/LibWeb/HTML/HTMLObjectElement.idl

@@ -1,3 +1,4 @@
+#import <DOM/Document.idl>
 #import <HTML/HTMLElement.idl>
 
 interface HTMLObjectElement : HTMLElement {