GDK W32: no "GDK_SELECTION" event for clipboard

_gdk_win32_display_convert_selection() does not return anything,
it generates a selection notify event instead. Depending on how
successful it was, the event will have property=GDK_NONE or
property="GDK_SELECTION".

property="GDK_SELECTION" is the default return value for successful
cases, and it tells GTK to grab the data that GDK previously deposited
using selection_property_store().

The problem is that the clipboard branch of this function calls
open_clipboard_timeout(), which can't return anything meaningful (it's
normally a timeout function), and thus doesn't know whether the function
succeeded or failed. Due to my oversight, this resulted in GDK
generating two selection notification events - one from inside of
open_clipboard_timeout() (with the right property, if successful),
and one from the catch-all last line (always defaulting to "GDK_SELECTION").

This caused issue #2223, where GTK only expected exactly one
notification per request, and got confused because it was getting two.

I've looked at the code in open_clipboard_timeout(), and it seems to me
that it always generates a notification (a successful one or an
unsuccessful one). Thus the branch of the function that calls it
directly does not need to follow up with a catch-all notification and
can just return.

This seems to be fixing issue #2223, at least for me, but i'm not
entirely sure that this will not have any adverse side-effects.
Clipboard handling in GTK3 is a complicated mess.

gdk/win32/gdkselection-win32.c

@@ -2251,6 +2251,7 @@ _gdk_win32_display_convert_selection (GdkDisplay *display,
 
           queue_open_clipboard (GDK_WIN32_CLIPBOARD_QUEUE_ACTION_CONVERT, display, requestor, target, time);
           open_clipboard_timeout (NULL);
+          return;
         }
       else
         {