Implement Manager#flush

lib/uh/wm/manager.rb

@@ -35,6 +35,10 @@ module Uh
         @events.emit :disconnected
       end
 
+      def flush
+        @display.flush
+      end
+
       def grab_key keysym, mod = nil
         mod_mask = KEY_MODIFIERS[@modifier]
         mod_mask |= KEY_MODIFIERS[mod] if mod