Draft: Refactor the View

Commit 1 as a preparation for View Restoration and MDS

Archive: Load the conversation around a message, not a timestamp
    
Message timestamps are not unique, so a message can be pushed out of the
window loaded around its own timestamp by others sharing it, and jumping
to it from the search or the activity list silently does nothing.
    
Pass the primary key as well and split the window exactly at it, so the
target is always the first row returned.

Commit 2 to simplify the logic

ConversationView: Track scroll position as a single intent
    
Store where the view wants to be instead, as "follow the bottom", "anchor a row" or "seek a row", and apply it in size_allocate() only.