Add Viewport#createSelectionBuffer()

Add Viewport#createSelectionBuffer()

Creates a Viewport_Selection object containing all elements currently in
the buffer.

Using this fixes generating the cached message list sent in viewport
requests.  Most notably, deleting messages in threaded mailboxes -
we were unnecessarily downloading all message data again even though it
was cached on the browser.  Instead, we just need to update the threaded
sort information on message deletion.