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/runtime/doc/popup.txt b/runtime/doc/popup.txt
index ba4dc3f..b0397a3 100644
--- a/runtime/doc/popup.txt
+++ b/runtime/doc/popup.txt
@@ -170,6 +170,7 @@
 Other:
 	|popup_getoptions()|	get current options for a popup
 	|popup_getpos()|	get actual position and size of a popup
+	|popup_locate()|	find popup window at a screen position
 
 
 DETAILS						*popup-function-details*
@@ -343,6 +344,13 @@
 		exists but is not a popup window an error is given. *E993*
 
 
+popup_locate({row}, {col})				 *popup_locate()*
+		Return the |window-ID| of the popup at screen positoin {row}
+		and {col}.  If there are multiple popups the one with the
+		highest zindex is returned.  If there are no popups at this
+		position then zero is returned.
+
+
 popup_menu({what}, {options})				 *popup_menu()*
 		Show the {what} near the cursor, handle selecting one of the
 		items with cursorkeys, and close it an item is selected with