Add ZBox, an implementation of stacked / modal Widgets

[cceckman]

This is an alternative to #103: a different way to have Widgets that overlay each other.

It defines a new ZBox, which is analogous to HBox and VBox, except that instead of constraining "same width" or "same height", it constrains "same size", and forces overwriting.

The Append API is slightly different, as well; rather than providing access to indices, it returns a "remover" function that can remove the just-added widget. This feels to me like an intuitive way to handle multi-dialog closure:

ui.Update(func() {
  d := NewCustomDialog()
  d.Remover= root.Append(d)
})

since indices may change if the dialogs aren't removed in stack order.

Some main differences from #103:

• It allows for multiple popups, on top of each other; see the added example in zbox_test.go.
• It is itself a Widget, so it can be used for not-the-entire-view dialogs as well- e.g. rendering an underlying view invisible without overwriting the whole screen.
• It's unbuffered; Draw delegates to all the Draw methods of its contents, serially. I haven't checked to see if this causes flickering / tearing / etc- that's a detail of tcell I haven't dug in to.

It follows #105, since it makes "box with transparent background" an ~easy way to do layout. (Maybe it's worth extracting center publicly- func Center(w Widget) Widget - if this is the approach we want to take?)