[QT] Don't ask for a passphrase to getnewaddress.

With an encrypted wallet the GUI was prompting for a passphrase every time
the user requested a new address. This is unnecessary, increases the
exposure to keyboard sniffers, and discourages using fresh addresses for
every transaction.

Instead only prompt for a passphrase when the keypool runs out, also call
the new address function with the flag that prevents reuse.

Thanks to AlexNagy on IRC for pointing this out and who wouldn't take any
lip from a curmudgeonly developer and insisted on what he knew to be true.