Implement niri msg pick-window

* feat: `niri msg pick-window`

* fixes

---------

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

niri-ipc/src/lib.rs

@@ -63,6 +63,8 @@ pub enum Request {
     FocusedOutput,
     /// Request information about the focused window.
     FocusedWindow,
+    /// Request picking a window and get its information.
+    PickWindow,
     /// Perform an action.
     Action(Action),
     /// Change output configuration temporarily.
@@ -129,6 +131,8 @@ pub enum Response {
     FocusedOutput(Option<Output>),
     /// Information about the focused window.
     FocusedWindow(Option<Window>),
+    /// Information about the picked window.
+    PickedWindow(Option<Window>),
     /// Output configuration change result.
     OutputConfigChanged(OutputConfigChanged),
 }