xcb: Always send dnd events to the proxy window if it exists

The standard http://www.newplanetsoftware.com/xdnd/

 says:
"The only place where the proxy window should be used is when
checking XdndAware and in the calls to XSendEvent()".

Change-Id: I62e504ba52ec7ec2a941c53a84889bb3eb4b4cc1
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>