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/component/input.cpp

@@ -200,7 +200,7 @@ class InputBase : public ComponentBase {
   bool OnEvent(Event event) override {
     wrapped_content_ = to_wstring(*content_);
     if (wrapped_input_.OnEvent(event)) {
-      content_ = to_string(wrapped_content_);
+      *content_ = to_string(wrapped_content_);
       return true;
     }
     return false;