layer: try to set keyboard focus on map

...for the following reasons:

1. We interpret 'normal input-focus semantics' for clients with on-demand
   keyboard interactivity to means that a surface receives input focus on
   cursor-button-press AND on map (the latter previously missing), just
   like a normal window would. In this regard, we do not differentiate
   between layers.

2. Most layer-surfaces set the keyboard interactivity at a similar time to
   their first (and normally only) map, so the absence of an explicit
   attempt to focus on map does not make a difference. However, for a
   long-running layer-shell client (such as lxqt-runner) which sets the
   interactivity on launch and then maps/unmaps many times throughout its
   lifetime, a specific focus-attempt is required on map to avoid the
   client itself having to keep resetting its interactivity to grab the
   keyboard on map.

3. Compositors like sway and river process focus (for clients with
   keyboard-interactivity)  in their map-handlers, so this makes for a
   common approach.

Fixes: #1653

src/layers.c

@@ -337,17 +337,16 @@ handle_map(struct wl_listener *listener, void *data)
 	if (wlr_output) {
 		output_update_usable_area(wlr_output->data);
 	}
+
 	/*
 	 * Since moving to the wlroots scene-graph API, there is no need to
 	 * call wlr_surface_send_enter() from here since that will be done
 	 * automatically based on the position of the surface and outputs in
 	 * the scene. See wlr_scene_surface_create() documentation.
 	 */
 
-	/*
-	 * Processing of keyboard-interactivity changes is done in the
-	 * commit-handler.
-	 */
+	struct seat *seat = &layer->server->seat;
+	layer_try_set_focus(seat, layer->scene_layer_surface->layer_surface);
 }
 
 static void