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.

examples/component/slider.cpp

@@ -7,7 +7,7 @@ using namespace ftxui;
 int main(int argc, const char* argv[]) {
   auto screen = ScreenInteractive::TerminalOutput();
   int value = 50;
-  auto slider = Slider(L"Value:", &value, 0, 100, 1);
+  auto slider = Slider("Value:", &value, 0, 100, 1);
   screen.Loop(slider);
 }