Gum overlay logic

I did some rebasing and used git push force and I managed to loose the reference to the original pull request #229.

Anyway I've fixed the changes based on the comments in #229 and made a clean pull request.

Sorry for the hassle.

Fixes #137 .
Btw instead of having this in imperative style with code.
Why don't you do it declarative on the html?
  <core-overlay id="gum-error-overlay" backdrop autoCloseDisabled> ?