Add childElements convenience method to scala.xml.Elem

[scabug]

I'd like a method like this on scala.xml.Elem

def childElements = child.collect { case e: Elem => e }

So that I can save a few keystrokes when I just want the children that are elements, not all of the children.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-3336?orig=1
Reporter: @eengbrec

[scabug]

@SethTisue said:
The scala-xml library is now community-maintained. Issues with it are now tracked at https://github.com/scala/scala-xml/issues instead of here in the Scala JIRA.

Interested community members: if you consider this issue significant, feel free to open a new issue for it on GitHub, with links in both directions.