Added menu item to unlock for minting only

I added a new menu item in "Settings" to decrypt the wallet for minting only. It asks for the passphrase and does the same as the `walletpassphrase` RPC command with `mintonly` enabled.

There is no timeout. To lock the wallet just use the menu item again.

The lock/unlock is effective immediately but the GUI may take a long time to update the status bar (just like the RPC command).