LibHTML: Add Comment and CharacterData nodes and improve HTML parsing

This patch adds the CharacterData subclass of Node, which is now the
parent class of Text and a new Comment class.

A Comment node is one of these in HTML: <!--hello friends-->
Since these occur somewhat frequently on the web, we need to be able
to parse them.

This patch also adds a child rejection mechanism to the DOM tree.
Nodes can now override is_child_allowed(Node) and return false if they
don't want a particular Node to become a child of theirs. This is used
to prevent Document from taking on unwanted children.

Libraries/LibHTML/Makefile.shared

@@ -17,6 +17,8 @@ LIBHTML_OBJS = \
     DOM/HTMLBlinkElement.o \
     DOM/HTMLBRElement.o \
     DOM/Document.o \
+    DOM/CharacterData.o \
+    DOM/Comment.o \
     DOM/Text.o \
     DOM/DocumentType.o \
     DOM/ElementFactory.o \