Preserve and restore x and y origin of editor viewport in Editor state.

Preserve and restore x and y origin of editor viewport in Editor state.
Clean up Editor's update of various things by:
1. merging Editor's handlers of SuperRapidScreenUpdate.
2. separating out work to be done on this update and work to be done
   on a locate.
Hopefully easier to understand this way.

git-svn-id: svn://localhost/ardour2/branches/3.0@6648 d708f5d6-7413-0410-9779-e7cbd77b26cf