WIP: Refactor Linux platform implementation (#10227)

This puts the Linux platform implementation at a similar code style and
quality to the macOS platform. The largest change is that I collapsed
the `LinuxPlatform` -> `[Backend]` -> `[Backend]State` ->
`[Backend]StateInner` to just `[Backend]` and `[Backend]State`, and in
the process removed most of the `Rc`s and `RefCell`s.

TODO:
- [x] Make sure that this is on-par with the existing implementation
- [x] Review in detail, now that the large changes are done.
- [ ] Update the roadmap

Release Notes:

- N/A

crates/workspace/src/persistence.rs

@@ -352,7 +352,8 @@ impl WorkspaceDb {
                 // Clear out panes and pane_groups
                 conn.exec_bound(sql!(
                     DELETE FROM pane_groups WHERE workspace_id = ?1;
-                    DELETE FROM panes WHERE workspace_id = ?1;))?(workspace.id)?;
+                    DELETE FROM panes WHERE workspace_id = ?1;))?(workspace.id)
+                .context("Clearing old panes")?;
 
                 conn.exec_bound(sql!(
                     DELETE FROM workspaces WHERE workspace_location = ? AND workspace_id != ?