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)
Merge pull request #2995 from Diapolo/GUI_message

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