Allow tracking focus change events in watchers

Fixes #2918

docs/changelog.rst

@@ -19,6 +19,8 @@ To update |kitty|, :doc:`follow the instructions <binary>`.
 - Add a new extensible escape code to allow terminal programs to trigger
   desktop notifications. See :ref:`desktop_notifications` (:iss:`1474`)
 
+- Allow tracking focus change events in watchers (:iss:`2918`)
+
 
 0.18.3 [2020-08-11]
 -------------------

docs/launch.rst

@@ -74,6 +74,9 @@ functions for the events you are interested in, for example:
     def on_resize(boss, window, data):
         # Here data will contain old_geometry and new_geometry
 
+    def on_focus_change(boss, window, data):
+        # Here data kill contain focused
+
     def on_close(boss, window, data):
         # called when window is closed, typically when the program running in
         # it exits.