Add -screenHeightDifferenceFrom4InchDisplay helper

64 bit CGFloat is double anyhow, so I think it's easier just to leave out the f from now on. In any case, I'm sure the compiler will handle simple literals like this well. I think the f just adds typing labor, and adds noise for the reader.

But, I'm closing this PR because it turns out that the screen bounds and key window bounds are incorrectly always set to 1024x768 in the resizable iPhone simulator that I'm using for the large screen size work, so I can't test this method properly yet. I'll open the PR again later once Apple fixes this.