Bitcoin-Qt: BitcoinGUI::message() updates/fixes
Bitcoin-Qt: BitcoinGUI::message() updates/fixes

- ensure message boxes are shown in center of our main window, not
  centered on the users desktop
- always prefer user supplied titles for message boxes over the functions
  defaults (fixes a bug, where transaction info messages did not contain
  information, if it was incoming or outgoing)

Rebased-from: ca3c4d8d61d67f29c5db2b526cbfb5a4e7d9b727 0.8.x

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