enhancement : validation icons

The validation icons do not show.
I fixed that using :after. But that needs crossbrowser testing.

You can see it here in action: http://redaktor.circinus.uberspace.de/schemer/
[e.g. put an invalid URI in the "id" field on the left]

I could stick a pull request together (with every detected [minor] issue) when above project is ready.
Fine for you?