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/dom/elements.hpp

@@ -20,7 +20,7 @@ using GraphFunction = std::function<std::vector<int>(int, int)>;
 // Pipe elements into decorator togethers.
 // For instance the next lines are equivalents:
 // -> text("ftxui") | bold | underlined
-// -> underlined(bold(text(L"FTXUI")))
+// -> underlined(bold(text("FTXUI")))
 Element operator|(Element, Decorator);
 Elements operator|(Elements, Decorator);
 Decorator operator|(Decorator, Decorator);

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.

include/ftxui/screen/string.hpp

@@ -1,8 +1,8 @@
 #ifndef FTXUI_SCREEN_STRING_HPP
 #define FTXUI_SCREEN_STRING_HPP
 
-#include <string>
-#include <vector>
+#include <string>  // for string, wstring, to_string
+#include <vector>  // for vector
 
 namespace ftxui {
 std::string to_string(const std::wstring& s);
@@ -13,13 +13,13 @@ std::wstring to_wstring(T s) {
   return to_wstring(std::to_string(s));
 }
 
-int wchar_width(wchar_t);
-int wstring_width(const std::wstring&);
 int string_width(const std::string&);
 std::vector<std::string> Utf8ToGlyphs(const std::string& input);
 
 }  // namespace ftxui
 
+#include "ftxui/screen/deprecated.hpp"
+
 #endif /* end of include guard: FTXUI_SCREEN_STRING_HPP */
 
 // Copyright 2020 Arthur Sonzogni. All rights reserved.