Add TOTP support

This adds support for time based one time passwords (issue #122 (closed)). It adds a new field to the entry page which allows you to specify a secret (no QR support yet but that could be added in the future). It also allows for copying of the time based code from both the entry page and the list of entries. A URI is stored in a custom "otp" field which is the same field and same fashion that keepassxc stores it so a database which is used by both applications will share OPT information.

This adds a new dependency pyotp which is used to generate the time based code and to parse/write the URIs.