Improve fatals and missing romset messages (also introducing ui_msgbox_header as a middle-of-the-road option)

src/win/win_dialog.c

@@ -50,6 +50,13 @@ ui_msgbox(int flags, void *message)
 }
 
 
+int
+ui_msgbox_header(int flags, void *header, void *message)
+{
+    return ui_msgbox_ex(flags, header, message, NULL, NULL, NULL);
+}
+
+
 int
 ui_msgbox_ex(int flags, void *header, void *message, void *btn1, void *btn2, void *btn3) {
     WCHAR temp[512];