Leave the Workspace to do the workspace parts, and extract the scrolling parts
into a new file. This is a pre-requisite for things like the floating layer
(which will live in a workspace alongside the scrolling layer).
As part of this huge refactor, I found and fixed at least these issues:
- Wrong horizontal popup unconstraining for a smaller window in an
always-centered column.
- Wrong workspace switch in focus_up_or_right().
Ivan Molodetskikh