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/proto/popupwin.pro b/src/proto/popupwin.pro
index 12be54f..1ff9950 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -57,6 +57,7 @@
 int popup_is_popup(win_T *wp);
 win_T *popup_find_preview_window(void);
 win_T *popup_find_info_window(void);
+void f_popup_findecho(typval_T *argvars, typval_T *rettv);
 void f_popup_findinfo(typval_T *argvars, typval_T *rettv);
 void f_popup_findpreview(typval_T *argvars, typval_T *rettv);
 int popup_create_preview_window(int info);
@@ -67,7 +68,7 @@
 void popup_show_message_win(void);
 int popup_message_win_visible(void);
 void popup_hide_message_win(void);
-void start_echowindow(void);
+void start_echowindow(int time_sec);
 void end_echowindow(void);
 int popup_win_closed(win_T *win);
 void popup_set_title(win_T *wp);