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.
This removes the requirement of having a global object that actually
inherits from JS::GlobalObject, which is now a perfectly valid scenario.
With the upcoming removal of wrapper objects in LibWeb, the HTML::Window
object will inherit from DOM::EventTarget, which means it cannot also
inherit from JS::GlobalObject.
This patch implements the "create a new browsing context" function from
the HTML spec and replaces our existing logic with it.
The big difference is that browsing contexts now initially navigate to
"about:blank" instead of starting out in a strange "empty" state.
This makes it possible for websites to create a new iframe and start
scripting inside it right away, without having to load an URL into it.
The way we've been creating DOM::Document has been pretty far from what
the spec tells us to do, and this is a first big step towards getting us
closer to spec.
The new Document::create_and_initialize() is called by FrameLoader after
loading a "text/html" resource.
We create the JS Realm and the Window object when creating the Document
(previously, we'd do it on first access to Document::interpreter().)
The realm execution context is owned by the Environment Settings Object.
This is now the source of truth for 'user enabled/disabled scripting',
but it has to ask the window's page, which actually stores the setting.
Also use this new functionality in two places where it was previously
marked as a FIXME.
The environment settings object is effectively the context a piece of
script is running under, for example, it contains the origin,
responsible document, realm, global object and event loop for the
current context. This effectively replaces ScriptExecutionContext, but
it cannot be removed in this commit as EventTarget still depends on it.
https://html.spec.whatwg.org/multipage/webappapis.html#environment-settings-object
