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.

runtime/doc/eval.txt

@@ -2535,6 +2535,7 @@ or({expr}, {expr})		Number	bitwise OR
 pathshorten({expr})		String	shorten directory names in a path
 perleval({expr})		any	evaluate |Perl| expression
 popup_atcursor({what}, {options}) Number create popup window near the cursor
+popup_beval({what}, {options}) 	Number 	create popup window for 'ballooneval'
 popup_clear()			none	close all popup windows
 popup_close({id} [, {result}])	none	close popup window {id}
 popup_create({what}, {options}) Number	create a popup window
@@ -2613,6 +2614,7 @@ screenattr({row}, {col})	Number	attribute at screen position
 screenchar({row}, {col})	Number	character at screen position
 screenchars({row}, {col})	List	List of characters at screen position
 screencol()			Number	current cursor column
+screenpos({winid}, {lnum}, {col}) Dict	screen row and col of a text character
 screenrow()			Number	current cursor row
 screenstring({row}, {col})	String	characters at screen position
 search({pattern} [, {flags} [, {stopline} [, {timeout}]]])
@@ -7907,6 +7909,23 @@ screencol()							*screencol()*
 			nnoremap <expr> GG ":echom ".screencol()."\n"
 			nnoremap <silent> GG :echom screencol()<CR>
 <
+screenpos({winid}, {lnum}, {col})				*screenpos()*
+		The result is a Dict with the screen position of the text
+		character in window {winid} at buffer line {lnum} and column
+		{col}.  {col} is a one-based byte index.
+		The Dict has these members:
+			row	screen row
+			col	first screen column
+			endcol	last screen column
+			curscol	cursor screen column
+		If the specified position is not visible, all values are zero.
+		The "endcol" value differs from "col" when the character
+		occupies more than one screen cell.  E.g. for a Tab "col" can
+		be 1 and "endcol" can be 8.
+		The "curscol" value is where the cursor would be placed.  For
+		a Tab it would be the same as "endcol", while for a double
+		width character it would be the same as "col".
+
 screenrow()							*screenrow()*
 		The result is a Number, which is the current screen row of the
 		cursor.  The top line has number one.