[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.