patch 8.1.1437: code to handle callbacks is duplicated

Problem:    Code to handle callbacks is duplicated.
Solution:   Add callback_T and functions to deal with it.
diff --git a/src/popupwin.c b/src/popupwin.c
index f828d91..73b98be 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -149,10 +149,10 @@
 	if (get_lambda_tv(&ptr, &tv, TRUE) == OK)
 	{
 	    wp->w_popup_timer = create_timer(nr, 0);
-	    wp->w_popup_timer->tr_callback =
+	    wp->w_popup_timer->tr_callback.cb_name =
 				  vim_strsave(partial_name(tv.vval.v_partial));
-	    func_ref(wp->w_popup_timer->tr_callback);
-	    wp->w_popup_timer->tr_partial = tv.vval.v_partial;
+	    func_ref(wp->w_popup_timer->tr_callback.cb_name);
+	    wp->w_popup_timer->tr_callback.cb_partial = tv.vval.v_partial;
 	}
     }
 #endif