Fixing simple-arc.rb and gray DSL method *picks up #394)

This PR is a continuation of #394 - I don't like it when PRs are open for too long - I want work to get in! =D

So I fixed it up e.g. the width is now the one of the app, as this seems to be the shoes3 behavior. However I kind of like the expected behavior from before which was actually calculating the width of an element. There were even specs for this, I put them on pending now and would then delete them/write new ones that assert that it's the width of the parent/app.

I also tried substituting the arc method there with creating a `Shoes::Arc` but that looked totally different probably due to the bug in #423

Would love feedback on this and want to get this in soon :-)