LibWeb: Add initial implementation of Element.blur()

This implementation includes a first cut at run the unfocusing steps
from the spec, with many things left unimplemented.

The viewport related spec steps in particular don't seem to map to
LibWeb concepts, which makes figuring out if things are properly focused
much more difficult.

Userland/Libraries/LibWeb/HTML/HTMLElement.idl

@@ -15,6 +15,8 @@ interface HTMLElement : Element {
     // FIXME: Support the optional FocusOptions parameter.
     undefined focus();
 
+    undefined blur();
+
     [LegacyNullToEmptyString] attribute DOMString innerText;
 
     readonly attribute long offsetTop;