Refactor teaser viewlet: use "available" instead of "getSimpleLayoutContents".

The result of getSimpleLayoutContents is static: either context or nothing.
This is an "available" check, not a search for (multiple) results.
I therefore refactor this to be more expressive and easier to understand.
Hide teaser viewlet for non-simplelayout views.
Merge pull request #120 from 4teamwork/jone-show-teaser-only-on-simplelayout-views

Hide teaser viewlet on non-simplelayout views.