Fix retrieving the selection/dnd data

Installing a roudtrip on the custom event queue in the wl_data_offer.offer
handler is broken because that triggers a wl_data_deveice.selection event,
which emits the QClipboard changed signal, so code listening to it may end
up trying to retrieve the clipboard data before the roundtrip ends.
Additionally, we're calling wl_data_offer.receive for each mime type, even
if then we never read from the fd, making the source client do work for no
reason. Instead, call wl_data_offer.receive retrieveData_sys, that is when
actually retreiving the data.
We don't need to install a roundtrip after that, just flushing out the
requests is enough, because we wait up to one second for the source client
to write into the fd.

Change-Id: I180779e375ebd5a22af7084458505a41107fab19