LibWeb: Remove inheritance of FormAssociatedElement from HTMLElement

HTMLObjectElement will need to be both a FormAssociatedElement and a
BrowsingContextContainer. Currently, both of these classes inherit from
HTMLElement. This can work in C++, but is generally frowned upon, and
doesn't play particularly well with the rest of LibWeb.

Instead, we can essentially revert commit 3bb5c62 to remove HTMLElement
from FormAssociatedElement's hierarchy. This means that objects such as
HTMLObjectElement individually inherit from FormAssociatedElement and
HTMLElement now.

Some caveats are:

* FormAssociatedElement still needs to know when the HTMLElement is
  inserted into and removed from the DOM. This hook is automatically
  injected via a macro now, while still allowing classes like
  HTMLInputElement to also know when the element is inserted.

* Casting from a DOM::Element to a FormAssociatedElement is now a
  sideways cast, rather than directly following an inheritance chain.
  This means static_cast cannot be used here; but we can safely use
  dynamic_cast since the only 2 instances of this already use RTTI to
  verify the cast.

Userland/Libraries/LibWeb/HTML/HTMLTextAreaElement.cpp

@@ -9,7 +9,7 @@
 namespace Web::HTML {
 
 HTMLTextAreaElement::HTMLTextAreaElement(DOM::Document& document, DOM::QualifiedName qualified_name)
-    : FormAssociatedElement(document, move(qualified_name))
+    : HTMLElement(document, move(qualified_name))
 {
 }