LibWeb: Don't include Window.h in Element.h and Document.h

Window.h is a rather heavy file, so let's try not to include it in
header files when we can!

Element.h now also includes LibWeb/Bindings/Intrinsics.h, but that's
just out of my laziness. Most if not all objects call
`Bindings::ensure_web_prototype<>()` anyway, so I don't think we would
gain much by sticking the header to source files instead.

Userland/Libraries/LibWeb/HTML/TraversableNavigable.cpp

@@ -12,6 +12,7 @@
 #include <LibWeb/HTML/NavigationParams.h>
 #include <LibWeb/HTML/SessionHistoryEntry.h>
 #include <LibWeb/HTML/TraversableNavigable.h>
+#include <LibWeb/HTML/Window.h>
 #include <LibWeb/Page/Page.h>
 #include <LibWeb/Platform/EventLoopPlugin.h>