patch 9.0.0321: cannot use the message popup window directly

Problem:    Cannot use the message popup window directly.
Solution:   Add ":echowindow".
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index b8cf41d..3ebbf23 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -548,6 +548,9 @@
 EXCMD(CMD_echon,	"echon",	ex_echo,
 	EX_EXTRA|EX_NOTRLCOM|EX_EXPR_ARG|EX_SBOXOK|EX_CMDWIN|EX_LOCK_OK,
 	ADDR_NONE),
+EXCMD(CMD_echowindow,	"echowindow",	ex_execute,
+	EX_EXTRA|EX_NOTRLCOM|EX_EXPR_ARG|EX_SBOXOK|EX_CMDWIN|EX_LOCK_OK,
+	ADDR_NONE),
 EXCMD(CMD_else,		"else",		ex_else,
 	EX_TRLBAR|EX_SBOXOK|EX_CMDWIN|EX_LOCK_OK|EX_WHOLE,
 	ADDR_NONE),