Polish up OverlayDialogFactory, esp. Modality

[rjrjr]

Should change buildDialogForView to accept ScreenViewHolder instead.

Probably should update OverlayDialogFactory to work more like ScreenViewFactory -- make DialogHolder public, and eliminate the separate update method.

Originally posted by @rjrjr in #712 (comment)

[rjrjr]

Should also rethink modality. At the moment the decision for the body to ignore events while dialogs are showing is hard coded into the BodyAndModalsContainer: FrameLayout. Seems like a clear mistake, since we'd really prefer that to feel like a property of the dialogs themselves.

Recall that we have a new marker interface for dialog-like renderings, Overlay. I'm thinking the thing to do is introduce a subtype, ModalOverlay : Overlay. (Thought about suggesting a val modal: Boolean, but allowing it to change on the fly would be a nightmare, I think.)