Use Layout from uh-layout as default

lib/uh/wm/runner.rb

@@ -10,14 +10,15 @@ module Uh
         end
       end
 
-      attr_reader :env, :events, :manager, :layout
+      extend Forwardable
+      def_delegator :@env, :layout
+
+      attr_reader :env, :events, :manager
 
       def initialize env, manager: nil, stopped: false
-        raise ArgumentError, 'missing env layout class' unless env.layout_class
         @env      = env
         @events   = Dispatcher.new
         @manager  = manager || Manager.new(@events)
-        @layout   = @env.layout_class.new
         @stopped  = stopped
       end
 
@@ -58,7 +59,7 @@ module Uh
 
       def register_layout_event_hooks
         @events.on(:connected) do |display|
-          @layout.register display
+          layout.register display
         end
       end