LibWeb: Focus the document element when unfocusing an element

Otherwise, the currently focused element remains focused.

Tests/LibWeb/Text/input/input-blur.html

@@ -0,0 +1,18 @@
+<input id=input type=text>
+<script src="include.js"></script>
+<script>
+    test(() => {
+        let input = document.getElementById("input");
+
+        input.addEventListener("focus", () => {
+            println("focus");
+        });
+
+        input.addEventListener("blur", () => {
+            println("blur");
+        });
+
+        input.focus();
+        input.blur();
+    })
+</script>