patch 8.1.1673: cannot easily find the popup window at a certain position

Problem:    Cannot easily find the popup window at a certain position.
Solution:   Add popup_locate().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 4ee0d5d..43c8028 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -781,6 +781,7 @@
     {"popup_getoptions", 1, 1, f_popup_getoptions},
     {"popup_getpos",	1, 1, f_popup_getpos},
     {"popup_hide",	1, 1, f_popup_hide},
+    {"popup_locate",	2, 2, f_popup_locate},
     {"popup_menu",	2, 2, f_popup_menu},
     {"popup_move",	2, 2, f_popup_move},
     {"popup_notification", 2, 2, f_popup_notification},