Add a TrackMouse(false) on ScreenInteractive to disable mouse events (#726)

When mouse events are enabled, it is not possible to select text in the
terminal and copy it somewhere else. This could be usefull for some
applications if they don't need to handle mouse events.

Add a function on the ScreenInteractive class to disable grabbing of
mouse events so that it is e.g. possible to select text in the user
interface. The function needs to be called on the screen object before
starting the application loop if such a behaviour is desired.

include/ftxui/component/screen_interactive.hpp

@@ -31,6 +31,9 @@ class ScreenInteractive : public Screen {
   static ScreenInteractive FitComponent();
   static ScreenInteractive TerminalOutput();
 
+  // Options. Must be called before Loop().
+  void TrackMouse(bool enable = true);
+
   // Return the currently active screen, nullptr if none.
   static ScreenInteractive* Active();
 
@@ -84,6 +87,8 @@ class ScreenInteractive : public Screen {
                     Dimension dimension,
                     bool use_alternative_screen);
 
+  bool track_mouse_= true;
+
   Sender<Task> task_sender_;
   Receiver<Task> task_receiver_;