Align the property / child determination in Element class

Problem/Motivation

Follow-up from comment by @alexpott #3172166-34: Element::properties() produces notices if given an array with integer keys.

The Element::child() must be the inverse of Element::property() - so let's make this true in code. Element::children() should also follow the same logic.

Proposed resolution

For Element::child()

   public static function child($key) {
-    return !isset($key[0]) || $key[0] != '#';
+    return !static::property($key);
   }

For "Element::children() we have if (is_int($key) || $key === '' || $key[0] !== '#') { I guess we're inlining this here for performance - which makes sense (up to a point - this stuff is called thousands of times so often the measuring makes it seem way way slower than it actually is). But regardless - we can still make this more efficient by doing if (!(is_string($key) && $key[0] !== '#')). "

"Note the performance arguments for Element::child() don't really hold based on core usage."

Remaining tasks

Make changes.
Does this also require checking for performance for Element::children()?

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet