Another approach to handling FF invisible iframe

I think this approach is probably the better way to go. It changes more lines in svg-edit though. I'm going to propose it to the svg-edit folks and see if they like it.

The idea is that handle all the places where errors occur when initializing in a invisible iframe in Firefox.
And then provide a way to re-run the feature tests and units configuration. This way the code can be used the same way as before in all cases. And if someone wants to handle the invisible iframe in Firefox they can re-run the feature test after the iframe is shown. For my simple testing it seems the re-running of the feature tests is not even necessary, but I'm sure something will be broken without doing so.

The first commit shows the places that need to handle the FF errors. 
The second commit shows the changes necessary to re-run the feature tests.