patch 8.1.1645: cannot use a popup window for a balloon

Problem:    Cannot use a popup window for a balloon.
Solution:   Add popup_beval().  Add the "mousemoved" property.  Add the
            screenpos() function.
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 6a4e4a6..34d60f1 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -720,6 +720,7 @@
 	cursor()		position the cursor at a line/column
 	screencol()		get screen column of the cursor
 	screenrow()		get screen row of the cursor
+	screenpos()		screen row and col of a text character
 	getcurpos()		get position of the cursor
 	getpos()		get position of cursor, mark, etc.
 	setpos()		set position of cursor, mark, etc.
@@ -1046,6 +1047,8 @@
 	popup_create()		create popup centered in the screen
 	popup_atcursor()	create popup just above the cursor position,
 				closes when the cursor moves away
+	popup_beval()		at the position indicated by v:beval_
+				variables, closes when the mouse moves away
 	popup_notification()	show a notification for three seconds
 	popup_dialog()		create popup centered with padding and border
 	popup_menu()		prompt for selecting an item from a list