Mouse support. Fix & verify Webassembly support.

There was some undefined behavior to be fixed in the terminal input
parser.

The behavior of flush seems to have change. The fix was to invert '\0'
and std::flush.

src/ftxui/screen/screen.cpp

@@ -184,7 +184,7 @@ std::string Screen::ToString() {
 }
 
 void Screen::Print() {
-  std::cout << ToString() << std::flush << (char)0;
+  std::cout << ToString() << '\0' << std::flush;
 }
 
 /// @brief Access a character a given position.