LibWeb: Add Node.textContent

This requires moving remove_all_children() from ParentNode to
Node, which makes ParentNode.cpp empty, so remove it.

It also co-opts the existing Node::text_content() method and
tweaks it slightly to fit the semantics of Node.textContent.

Libraries/LibWeb/CMakeLists.txt

@@ -33,7 +33,6 @@ set(SOURCES
     DOM/EventListener.cpp
     DOM/EventTarget.cpp
     DOM/Node.cpp
-    DOM/ParentNode.cpp
     DOM/Position.cpp
     DOM/TagNames.cpp
     DOM/Text.cpp