Flatten the namespaces.

Remove:
* ftxui::screen
* ftxui::dom
* ftxui::component

Keep:
* ftxui

ftxui/include/ftxui/component/component_direction.hpp

@@ -4,7 +4,6 @@
 #include "ftxui/component/component.hpp"
 
 namespace ftxui {
-namespace component {
 
 // A component where focus and events are automatically handled for you.
 // Please use ComponentVertical or ComponentHorizontal.
@@ -20,7 +19,6 @@ class ComponentDirection : public Component {
   Component* active_child_;
 };
 
-} // namespace component
 } // namespace ftxui
 
 #endif /* end of include guard: FTXUI_COMPONENT_COMPONENT_DIRECTION_H_ */