Implement `layout' run control keyword

Allow configuration of the layout by specifying either a class or an
instance.

lib/uh/wm/env.rb

@@ -21,8 +21,8 @@ module Uh
       def_delegators  :@output, :print, :puts
 
       attr_reader   :output, :keybinds
-      attr_accessor :verbose, :debug, :rc_path, :layout_class, :modifier,
-                    :worker
+      attr_accessor :verbose, :debug, :rc_path, :modifier, :worker,
+                    :layout, :layout_class
 
       def initialize output
         @output   = output

lib/uh/wm/run_control.rb

@@ -32,6 +32,14 @@ module Uh
         @env.keybinds[translate_keysym *keysyms] = block
       end
 
+      def layout obj
+        if obj.is_a? Class
+          @env.layout_class = obj
+        else
+          @env.layout = obj
+        end
+      end
+
       def worker type, **options
         @env.worker = [type, options]
       end