patch 8.1.1548: popup_dialog() is not implemented

Problem:    Popup_dialog() is not implemented.
Solution:   Implement popup_dialog() and popup_filter_yesno().
diff --git a/src/structs.h b/src/structs.h
index 5925927..09158a0 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1998,7 +1998,8 @@
 
 # define POPUPWIN_DEFAULT_ZINDEX	 50
 # define POPUPMENU_ZINDEX		100
-# define POPUPWIN_NOTIFICATION_ZINDEX   200
+# define POPUPWIN_DIALOG_ZINDEX		200
+# define POPUPWIN_NOTIFICATION_ZINDEX   300
 #endif
 
 /*