feat: desktop environment improvements

programs/include/gui/window.hpp

@@ -25,6 +25,7 @@ using WindowDrawCallback  = void (*)(Window* win, Framebuffer& fb);
 using WindowMouseCallback = void (*)(Window* win, MouseEvent& ev);
 using WindowKeyCallback   = void (*)(Window* win, const Zenith::KeyEvent& key);
 using WindowCloseCallback = void (*)(Window* win);
+using WindowPollCallback  = void (*)(Window* win);
 
 struct Window {
     char title[MAX_TITLE_LEN];
@@ -48,6 +49,7 @@ struct Window {
     WindowMouseCallback on_mouse;
     WindowKeyCallback   on_key;
     WindowCloseCallback on_close;
+    WindowPollCallback  on_poll;
     void* app_data;
 
     Rect titlebar_rect() const {