Remove Prerender constraint from UI type alias

[mightybyte]

A lot of the semantic-reflex widgets don't need the Prerender constraint, but they use the UI type alias so they get it. The result is that the user can't use these widgets in many places completely unnecessarily.

[tomsmalley]

The Prerender constraint is necessary due to transitions needing MonadIO for delaying events.

What are the situations that prevent you from making use of the prerender constraint?

[tomsmalley]

It would be nice to be able to drop that constraint where transitions aren't being used, though.

[mightybyte]

Yes. I don't object to the constraint being there where needed. I just think it shouldn't be there across the board.