conversation view zooming

Zoom is enabled on the entire conversation using the built-in
WebView mechanism.

Wide messages are individually best-effort shrunken to
fit-width.

Start a custom ViewGroup to hold both message headers and the
single WebView. A common view parent makes input handling easy:
the WebView drives scroll position, and position is relayed
to the custom ViewGroup to cheaply shift around headers without
a full relayout. For now, all headers are inflated at once, but
soon they can be recycled depending on the virtual scroll
viewport.

Change-Id: I92555f9b79e10630457b17ca970ab9a2e9028e80
16 files changed