Set `F = ()` on widgets with callbacks

[bvssvni]

This will make it optional to specify callback for a widget.

[mitchmindtree]

Hmmm I'm still getting

error: unable to infer enough type information about `_`; type annotations required [E0282]
 TextBox::new(tbox_uiid, text)                                                             
 ^~~~~~~~~~~~                                                                              

I think this might be because in the end, every widget calls .draw and each implementation of Drawable constrains F so that it must be some kind of FnMut, as the callback (Option<F>) is matched within .draw.

Perhaps we should consider a dummy type that impls the necessary FnMut to use as a default?

[bvssvni]

Alternative is to use a trait that is auto implemented for all types implementing FnMut, which we now can implement for () because where clauses can be used as refinements. Example http://is.gd/KsDFYo