Workspace back and forth (#253)

* implement workspace back and forth

* Make our own ID counter instead of SerialCounter, use a newtype

* Rename FocusWorkspaceBackAndForth to FocusWorkspacePrevious

* Add focus-workspace-previous to tests

* Don't special case in switch_workspace_previous

* Minor clean up

* Add switch_workspace_auto_back_and_forth to tests

* Skip animation on switch_workspace_previous

* Preserve previous_workspace_id on workspace movement

* Make Workspace::id private with a getter

Reduce the chance it gets overwritten.

* Add test for workspace ID uniqueness

* Update previous workspace ID upon moving workspace across monitors

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Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

niri-ipc/src/lib.rs

@@ -123,6 +123,8 @@ pub enum Action {
         #[cfg_attr(feature = "clap", arg())]
         index: u8,
     },
+    /// Focus the previous workspace.
+    FocusWorkspacePrevious,
     /// Move the focused window to the workspace below.
     MoveWindowToWorkspaceDown,
     /// Move the focused window to the workspace above.