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/dom/hflow.cpp

@@ -68,8 +68,8 @@ class HFlow : public Node {
 ///
 /// ```cpp
 /// hbox({
-///   text(L"Left"),
-///   text(L"Right"),
+///   text("Left"),
+///   text("Right"),
 /// });
 /// ```
 Element hflow(Elements children) {