wallet: Allow user to navigate options while encrypting at creation

[hernanmarino]

This fixes https://github.com/bitcoin-core/gui/issues/394. It adds a "Go back" button to the "Confirm wallet encryption" window, allowing the users to change the password if they want to. It also adds a Cancel button to the "Wallet to be encrypted" window. Prior to this change users had no option to alter the password, and were forced to either go ahead with wallet creation or cancel the whole process. Also, at the final window, they were shown a warning but with no option to cancel. The new workflow for wallet encryption and creation is similar to the following: