Introduce VirtualMethods::children_changed()

This virtual method mimics the behaviour of mutation observers and make it more
viable than the older child_inserted(), which didn't cover removed nodes and
was called as many times as there were inserted nodes.

A few other shortcomings where remove_child() was called directly instead of
Node::remove() were also fixed while at it.

components/script/lib.rs

@@ -19,6 +19,7 @@
 #![feature(nonzero)]
 #![feature(owned_ascii_ext)]
 #![feature(plugin)]
+#![feature(ref_slice)]
 #![feature(rc_unique)]
 #![feature(slice_chars)]
 #![feature(str_utf16)]