An open dialog can now take the pad the way it takes host text: a pad
sink it sets, ImePublishPad_nid_postfix forwarding each pad sample to
it, and ImeCapturesPad_nid_postfix telling the pad's owner that the
dialog has it. A keyboard view provider lets the presentation thread
read the text, caret and key grid it draws. Each is modelled on the
host-text sink: an atomic pointer the dialog sets and clears.
