add Widget module

runtime/lib/widget.rb

@@ -0,0 +1,33 @@
+module Widget
+  def initialize
+    @x = 0
+    @y = 0
+    @width = 0
+    @height = 0
+  end
+
+  def children
+    []
+  end
+
+  def resize(x, y, width, height)
+    @x = x
+    @y = y
+    @width = width
+    @height = height
+  end
+
+  def draw
+    if @x > 0 and @y > 0
+      render_begin
+      render
+      children.each do |child|
+        child.draw
+      end
+      render_end
+    end
+  end
+
+  def render_end
+  end
+end

runtime/lib/window.rb

@@ -1,6 +1,8 @@
 require "set"
 
 class Window
+  include Widget
+
   @windows = Set.new
 
   class << self