GDK W32: Ensure that selection request is processed

GDK W32: Ensure that selection request is processed

To do that, run the message loop for one second or until the side-effect
of running the selection request handler is achieved (as opposed to
running it until the event is no longer queued).

The disavantage of this method is that if the event handling is
somehow missed (due to a variety of reasons - after all, it's not
a straight path from an event being queued to property_change()
being called), this will loop for one second. Since we do process
events during that time, this will not hang the application, but
might still restrict some of the functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=786509