Update examples to use std::string. (#182)

In examples and tests, use std::string.

In addtion:
1. Address follow-up from:
https://github.com/ArthurSonzogni/FTXUI/pull/179
2. Fix a bug when Input is used with std::string.

include/ftxui/screen/deprecated.hpp

@@ -0,0 +1,15 @@
+#ifndef FTXUI_SCREEN_DEPRECATED_HPP
+#define FTXUI_SCREEN_DEPRECATED_HPP
+
+#include <string>
+
+namespace ftxui {
+int wchar_width(wchar_t);
+int wstring_width(const std::wstring&);
+}  // namespace ftxui
+
+#endif /* end of include guard: FTXUI_SCREEN_DEPRECATED_HPP */
+
+// Copyright 2021 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.