Handle really global selection offers in wayland clipboard.

Handle really global selection offers in wayland clipboard.

Handle selection offer globals properly even if they arrive during the
initial blocking read. If such a global arrives so early, the platform
integration is not yet available from QApplicationPrivate (as it is
just being constructed). Therefore the handling of the selection
offer has to be delayed. At the moment selection offers are typically
posted as globals and not added to the global list, but that is likely
to change in the future.

Change-Id: Ib4ae804ad7f19e05978ee08828b88e028a3bf7b2
Reviewed-on: http://codereview.qt.nokia.com/446
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>