add BufferWidget

runtime/lib/buffer_widget.rb

@@ -0,0 +1,5 @@
+require "widget"
+
+class BufferWidget
+  include Widget
+end

runtime/lib/window.rb

@@ -1,4 +1,5 @@
 require "set"
+require "widget"
 
 class Window
 

runtime/main.rb

@@ -3,6 +3,7 @@ def init_loadpath
 end
 
 def load_lib_files
+  require "buffer_widget"
   require "gl"
   require "gl_program"
   require "runtime"