layout: Extract ScrollingSpace

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().

niri-visual-tests/src/cases/layout.rs

@@ -2,7 +2,7 @@ use std::collections::HashMap;
 use std::time::Duration;
 
 use niri::animation::Clock;
-use niri::layout::workspace::ColumnWidth;
+use niri::layout::scrolling::ColumnWidth;
 use niri::layout::{ActivateWindow, LayoutElement as _, Options};
 use niri::render_helpers::RenderTarget;
 use niri_config::{Color, FloatOrInt, OutputName};