Addition of Import Private Key Menu

Does what it says on the tin.
Has full validation of private key before trying to import.
Includes logic to request passphrase if wallet is locked.
Include progress bar when scanning transactions as can take a few mins.

Motivation was to make it easier for people to import vanity addresses.
I wrote bitcoinvanity.appspot.com and so want to make importing key easier for users.

1st attempt at QT programming so could be improved I expect.