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().

runtime/doc/builtin.txt

@@ -396,6 +396,7 @@ popup_create({what}, {options}) Number	create a popup window
 popup_dialog({what}, {options}) Number	create a popup window used as a dialog
 popup_filter_menu({id}, {key})  Number	filter for a menu popup window
 popup_filter_yesno({id}, {key}) Number	filter for a dialog popup window
+popup_findecho()		Number	get window ID of popup for `:echowin`
 popup_findinfo()		Number	get window ID of info popup window
 popup_findpreview()		Number	get window ID of preview popup window
 popup_getoptions({id})		Dict	get options of popup window {id}

runtime/doc/eval.txt

@@ -3392,7 +3392,7 @@ text...
 			when the screen is redrawn.
 
 					*:echow* *:echowin* *:echowindow*
-:echow[indow] {expr1} ..
+:[N]echow[indow] {expr1} ..
 			Like |:echomsg| but when the messages popup window is
 			available the message is displayed there.  This means
 			it will show for three seconds and avoid a
@@ -3400,6 +3400,9 @@ text...
 			that, press Esc in Normal mode (when it would
 			otherwise beep).  If it disappears too soon you can
 			use `:messages` to see the text.
+			When [N] is given then the window will show up for
+			this number of seconds.  The last `:echowindow` with a
+			count matters, it is used once only.
 			The message window is available when Vim was compiled
 			with the +timer and the +popupwin features.
 

runtime/doc/popup.txt

@@ -347,6 +347,12 @@ popup_filter_yesno({id}, {key})				*popup_filter_yesno()*
 		See the example here: |popup_dialog-example|
 
 
+popup_findecho()					*popup_findecho()*
+		Get the |window-ID| for the popup that shows messages for the
+		`:echowindow` command.  Return zero if there is none.
+		Mainly useful to hide the popup.
+
+
 popup_findinfo()					*popup_findinfo()*
 		Get the |window-ID| for the popup info window, as it used by
 		the popup menu.  See |complete-popup|.  The info popup is

runtime/doc/usr_41.txt

@@ -1289,6 +1289,7 @@ Popup window:					*popup-window-functions*
 	popup_filter_yesno()	block until 'y' or 'n' is pressed
 	popup_getoptions()	get current options for a popup
 	popup_getpos()		get actual position and size of a popup
+	popup_findecho()	get window ID for popup used for `:echowindow`
 	popup_findinfo()	get window ID for popup info window
 	popup_findpreview()	get window ID for popup preview window
 	popup_list()		get list of all popup window IDs