Ensure that GUI2 windows trigger the redraw-events when closed

This changes the behaviour of the GUI2 draw-layers to rely on
tdistributor instead of thandler. tdistributor is used by every GUI2
window, making it the correct choice for triggering a redraw when it
is destroyed.

src/gui/auxiliary/event/dispatcher.hpp

@@ -22,6 +22,8 @@
 
 #include "SDL_events.h"
 
+#include "video.hpp"
+
 #include "utils/boost_function_guarded.hpp"
 #include <boost/mpl/int.hpp>
 #include <boost/utility/enable_if.hpp>

src/gui/auxiliary/event/distributor.hpp

@@ -271,6 +271,19 @@ public:
 	void keyboard_remove_from_chain(twidget* widget);
 
 private:
+	class layer : public video2::draw_layering
+	{
+	public:
+		virtual void handle_event(const SDL_Event& ) {}
+	#if SDL_VERSION_ATLEAST(2, 0, 0)
+		virtual void handle_window_event(const SDL_Event& ) {}
+	#endif
+		layer() : video2::draw_layering(false) { }
+	};
+
+	// make sure the appropriate things happens when we close.
+	layer layer_;
+
 #if 0
 	bool hover_pending_;			   /**< Is there a hover event pending? */
 	unsigned hover_id_;                /**< Id of the pending hover event. */

src/gui/auxiliary/event/handler.cpp

@@ -121,7 +121,7 @@ static Uint32 timer_sdl_poll_events(Uint32, void*)
  *
  * It's a new experimental class.
  */
-class thandler : public video2::draw_layering
+class thandler : public events::sdl_handler
 {
 	friend bool gui2::is_in_dialog();
 
@@ -298,7 +298,7 @@ private:
 };
 
 thandler::thandler()
-	: video2::draw_layering(false)
+	: events::sdl_handler(false)
 	, mouse_focus(NULL)
 	, dispatchers_()
 	, keyboard_focus_(NULL)