Fixed: couldn't set the first responder of a sheet

Previously the sheet was made key at the beginning of the animation. It turns out events would not reliably make it to the sheet during animation. The other problem is that if a text field was the first responder, at the beginning of the animation it is offscreen, which means it refuses first responder and sets the first responder to nil.

Now we no longer try to make the sheet key before the animation ends, avoiding the text field problem.