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.

src/ftxui/screen/string.cpp

@@ -13,6 +13,8 @@
 #include <locale>    // for wstring_convert
 #include <string>    // for string, basic_string, wstring, allocator
 
+#include "ftxui/screen/deprecated.hpp"  // for wchar_width, wstring_width
+
 namespace {
 
 struct Interval {