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/container_test.cpp

@@ -11,7 +11,7 @@
 using namespace ftxui;
 
 Component Focusable() {
-  return Button(L"", [] {});
+  return Button("", [] {});
 }
 Component NonFocusable() {
   return Container::Horizontal({});