HOTP counter validator

The "Add OTP..." form contains an input field for the counter value in HOTP token scheme. Currently input from this field is not properly validated: it only allows up to 4 digits, whereas liboath supports 64 bits and the specification does not impose any size limit at all. The UI field should at least be able to assume the full range permitted by our HOTP implementation.

Fix this by introducing a new custom validator type and using it in the QML UI.