Adds InputWindowElement.

src/Frame.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <string>
 #include <vector>
 
 #include "iDrawable.h"
@@ -46,6 +47,8 @@ class Frame : public iDrawable, public iWritable {
     }
   }
 
+  std::string input;
+
  private:
   std::vector<const iDrawable*> draw_;
   std::vector<const iWritable*> write_;