Detect invalid passwords and support retry

Keysmith will now detect incorrect passwords entered while unlocking accounts and prompt the user with appropriate error feedback. Additionally, password entry in the UI is now temporarily disabled while Keysmith tries to derive the key: this UI transition should help indicate to the user that "something is happening, please be patient"

To make this work a breaking change is introduced for the accounts data stored on disk. As a result, users will have to discard old ~/.config/org.kde.keysmith/Keysmith.conf files and redo their existing accounts setup.