Sections: added a notebook container to 'GetKeySection' to hold sign pages.

The notebook will display the following pages:
 - ScanFingerprint() page
 - SignKeyPage() page
 - PostSignPage() page
SignPages: added code for ScanFingerprintPage and SignKeyPage.

More work on implementing the GUI that offers a guided way to scan/type a fingerprint, download the key and sign it.
SignPages: added code for PostSignPage
GetKeySection: added a back button