patch 8.1.1071: cannot get composing characters from the screen

Problem:    Cannot get composing characters from the screen.
Solution:   Add screenchars() and screenstring(). (partly by Ozaki Kiichi,
            closes #4059)

runtime/doc/eval.txt

@@ -2525,8 +2525,10 @@ round({expr})			Float	round off {expr}
 rubyeval({expr})		any	evaluate |Ruby| expression
 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
 screenrow()			Number	current cursor row
+screenstring({row}, {col})	String	characters at screen position
 search({pattern} [, {flags} [, {stopline} [, {timeout}]]])
 				Number	search for {pattern}
 searchdecl({name} [, {global} [, {thisblock}]])
@@ -7510,6 +7512,13 @@ screenchar({row}, {col})					*screenchar()*
 		This is mainly to be used for testing.
 		Returns -1 when row or col is out of range.
 
+screenchars({row}, {col})					*screenchars()*
+		The result is a List of Numbers.  The first number is the same
+		as what |screenchar()| returns.  Further numbers are
+		composing characters on top of the base character.
+		This is mainly to be used for testing.
+		Returns an empty List when row or col is out of range.
+
 screencol()							*screencol()*
 		The result is a Number, which is the current screen column of
 		the cursor. The leftmost column has number 1.
@@ -7531,6 +7540,14 @@ screenrow()							*screenrow()*
 
 		Note: Same restrictions as with |screencol()|.
 
+screenstring({row}, {col})					*screenstring()*
+		The result is a String that contains the base character and
+		any composing characters at position [row, col] on the screen.
+		This is like |screenchars()| but returning a String with the
+		characters.
+		This is mainly to be used for testing.
+		Returns an empty String when row or col is out of range.
+
 search({pattern} [, {flags} [, {stopline} [, {timeout}]]])	*search()*
 		Search for regexp pattern {pattern}.  The search starts at the
 		cursor position (you can use |cursor()| to set it).

runtime/doc/usr_41.txt

@@ -723,6 +723,8 @@ Cursor and mark position:		*cursor-functions* *mark-functions*
 	diff_filler()		get the number of filler lines above a line
 	screenattr()		get attribute at a screen line/row
 	screenchar()		get character code at a screen line/row
+	screenchars()		get character codes at a screen line/row
+	screenstring()		get string of characters at a screen line/row
 
 Working with text in the current buffer:		*text-functions*
 	getline()		get a line or list of lines from the buffer