patch 9.0.0683: cannot specify a time for :echowindow

Problem:    Cannot specify a time for :echowindow.
Solution:   A count can be used to specify the display time. Add
            popup_findecho().
diff --git a/src/eval.c b/src/eval.c
index 342f93c..c37765f 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -6916,7 +6916,7 @@
 	else if (eap->cmdidx == CMD_echowindow)
 	{
 #ifdef HAS_MESSAGE_WINDOW
-	    start_echowindow();
+	    start_echowindow(eap->addr_count > 0 ? eap->line2 : 0);
 #endif
 	    msg_attr(ga.ga_data, echo_attr);
 #ifdef HAS_MESSAGE_WINDOW