[Qt] seed OpenSSL PNRG with Windows event data

- see https://bitcointalk.org/index.php?topic=113496.msg1228193#msg1228193
  for the initial suggestion for this
Right, but there is still overhead to calling this function for every event, even if the message is generated then discarded. This is a very low-level function and performance is important here.
What I'm afraid of are situations like that at some point the entropy pool runs out and suddenly the interface becomes lots slower.