Bitcoin-Qt: remove " - " on empty title in BitcoinGUI::message()

- don't display "Bitcoin - " when no title was supplied but just "Bitcoin"
qt: move export button to tabs

Having the export button at the top was confusing people into thinking
the entire wallet was exported.

This commit moves the export button to the address book, receiving
addresses and transaction tabs separately.
Merge pull request #2515 from laanwj/2013_04_move_exportbutton

qt: move export button to tabs
Merge pull request #2492 from Diapolo/Qt_message

Bitcoin-Qt: remove " - " on empty title in BitcoinGUI::message()