Implemented SDL event listener class and HUD toggle key (#4)

* Implemented SDL event listener class

* Add HUD toggle.

* frontend_listener: clean-up

* window: invoke all listener callbacks at once

* window: use raw pointers for listeners

* Rename WindowListener to SDLEventListener, reduce virtual functions

---------

Co-authored-by: RadiantDerg <jayvier13@gmail.com>

UnleashedRecomp/ui/window.cpp

@@ -1,4 +1,5 @@
 #include "window.h"
+#include "sdl_listener.h"
 #include <config.h>
 #include <kernel/function.h>
 #include <SDL_syswm.h>
@@ -107,6 +108,9 @@ int Window_OnSDLEvent(void*, SDL_Event* event)
         }
     }
 
+    for (auto listener : Window::s_eventListeners)
+        listener->OnSDLEvent(event);
+
     return 0;
 }