Add focus-window-previous action (#811)

* Add `FocusWindowPrevious` action

* remove [`

* track previous focus in Niri instead of every window

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

niri-ipc/src/lib.rs

@@ -204,6 +204,8 @@ pub enum Action {
         #[cfg_attr(feature = "clap", arg(long))]
         id: u64,
     },
+    /// Focus the previously focused window.
+    FocusWindowPrevious {},
     /// Focus the column to the left.
     FocusColumnLeft {},
     /// Focus the column to the right.