river-layout: add user_command_tags event

It is not guaranteed that the next layout_demand event after a user_command
event has the same active tags (for example when there are no views visible).
As an example, a user could trigger a user_command while no views are visible,
then switch to a different tag set which has active views. The active tags of
the previous layout_demand may also be different.

Therefore it is impossible to correctly implement a layout generator which has
user commands apply only to the currently active tag set, which is solved by
this patch.

rivertile/main.zig

@@ -298,6 +298,7 @@ const Output = struct {
                     .bottom => layout.commit("rivertile - bottom", ev.serial),
                 }
             },
+            .user_command_tags => {},
         }
     }
 };