Add a window swap operation (#899)

Swap the active window with the a neighboring column's active window.


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Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>
Take into account PR comments

- no longer behave like an expel when a swap is made in a direction
  where there is no column to swap with
- fix janky animation

niri-ipc/src/lib.rs

@@ -290,6 +290,10 @@ pub enum Action {
     ConsumeWindowIntoColumn {},
     /// Expel the focused window from the column.
     ExpelWindowFromColumn {},
+    /// Swap focused window with one to the right
+    SwapWindowRight {},
+    /// Swap focused window with one to the left
+    SwapWindowLeft {},
     /// Center the focused column on the screen.
     CenterColumn {},
     /// Center a window on the screen.